I have to contradict @saj14saj here. I have frequently had bread made with underdeveloped gluten (my grandma uses AP flour and tends to knead very short, 2-3 minutes per hand, and use very short proofing times). The bread is soft and cakelike, but it has no trouble rising, and it is neither flat nor dense.
On the other hand, I have had bread with exactly the same symptoms as yours - first feeling great, then left out for a long time to proof. After that, it looks good, but one touch makes it collapse into itself. The reason was very clear: overproofing. There is no doubt that underdeveloped gluten cannot have been a factor in my case. First, I am experienced enough to know when my gluten is developed - the bread was kneaded well beyond windowpane test. Second, it was a large batch of dough. I baked the first loaf at the optimal time and it rose just fine and had a nice texture with traceable gluten sheets through the crumb. It was the second loaf, which I baked a few hours later (and the proofing loaf spent them in a 30 degrees celsius kitchen in summer) which made the trouble. The dough had exactly the "loose" feel you describe, unlike the normal, springy feel before the proofing. It collapsed on touch and would not rise at all. It also had a very strong yeast fermentation taste, unlike the other loaf.
From your description and my experience, my conclusion is that overproofing until your starter died in its own waste products is the most likely culprit. The simple answer would be to not let it sit out overnight. The right amount of time to let it sit would depend on the room temperature, on the amount of starter you used, but also its leavening strength, and that is a bit hard to judge for a newly created starter. My best suggestion is to use trial and error and maybe bake 4 hours after knocking instead of 8 next time, and the time after that adjusting with a smaller increment in the right direction depending on whether the bread turns out overproofed or underrisen.
It could be an issue with how long you let it rise or maybe how warm you rested it at. I was making a deep dish pizza at home once and the recipe called for resting the dough in the fridge (mixed with the yeast and other ingredients), for 4hrs minimum. Due to my getting home a little after 5pm from work and some of my family leaving for work at 6:30pm, during the time I was attempting to make this, I couldn't use the suggested times for the different steps of the recipe. To shorten that time, I made the dough the day before. I let the dough rest in the fridge from about 9pm until 5:30pm the next day. When I made the pizzas the dough was very good, but it tasted a lot like sourdough, and that was using instant yeast out of a packet. I've since made the recipe with the correct resting times and it's good dough, but there is no sour taste.
So a long resting time for your bread, or possibly having it warmer might induce the yeast to process their 'food' faster. I know it's not true sourdough as the sourness there comes from a combination of yeast and lactobacilli's, but I did get a similar taste from the instant yeast packet and it was because of the duration I let the yeast work. Some of my family actually prefers the sourdough taste of that dough, so I sometimes make it that way. It also saves me from having to do too much work on any given day.
You can test my theory out and see if temp or the leavening time affects the taste. While you didn't originally want that particular flavor, you may come to enjoy it.
Best Answer
There are many ways to achieve a “pure” strain of yeast or other microorganisms. Modern technology with sterile environments are today’s method of choice, but another is to create an environment that is especially favorable for one kind and allow this to outcompete all others. The principle is not limited to yeast, another important field is cheese making when the raw cheese is aged in an environment with a specific mold - Roquefort being the classic example. In this case, humans use the already existing colonies in a specific environment or ecological niche.
Even sourdough today works in a similar way, but for sourdough the baker creates a stable mix of yeast(s) and lactobacillae - and by adjusting hydration and temperatures can even regulate acidity and yeast activity.
The earliest bread yeast was a by-product of brewing (hence the name Saccharomyces cerevisiae). And brewers, especially those that brewed on a large scale like monasteries and commercial brewers, carefully guarded and cultivated their yeast strains that had developed over time. The yeast was and is a huge factor for the flavor of the beer. This lead to the first “just yeast” leaveners. You could still obtain some barm from a hobby brewer and give it a try. Ensure that they are brewing with a top-cropping yeast like S. cervisiae, not a bottom-cropping yeast like S. pastorianus.
The main difference between “wild yeast” and sourdough is ratios: you can use for example raisins soaked in water to get a yeast-rich liquid as opposed to a classic sourdough. Elderflower is also rich in wild yeasts. In short, all plants that ferment easily are suitable sources for wild yeast. I doubt that setting up near a tree will have much of an effect.