Bread – Why did flatbread dominate the Middle East but Europe adopted raised breads

There shouldn't be any significant difference between an established homemade sourdough culture and one that's seeded from something you bought (I'm assuming like the culture that King Arthur Flour Co sells online). In fact, no matter where you bought your starter culture, over time the local bacterial flora would crowd out the bacteria that was in the culture you bought. But, I'm sure the mold infection had nothing to do with rogue bacteria. (Bad bacteria can spoil a starter too, but that would be a different set of symptoms from the green carpet on top.) It could be that your starter didn't get well established, but it doesn't sound like that was the problem if you successfully made some bread from it.

In a sourdough culture, the acid produced by the bacteria and alcohol produced by the yeast make an environment that's somewhat resitant to bad bacteria (like salmonella) or mold. But the culture needs to be fairly active to maintain its resistance. After two weeks in a warm fridge, your yeast would be dormant and the bacteria would probably have run rampant for a while (after the yeast went dormant) and then started to also die off.

In a warm fridge you could maybe go a week without feeding the culture. At room temperature it's one day optimally and two days max. The best way to avoid infection is to have two small covered containers about 1½ to 2 cups in size. To make a new generation of your starter, begin with a clean container, add equal parts by weight of the previous generation of starter, flour and water. So for example, 25 grams of starter, plus 25 grams of flour and 25 of water. Mix it up and put it in the fridge. In one week, get the other container, clean it out, and do the same thing over again.

I always keep the old generation in its container as a backup (in case of a mold or other infection). You just need to keep track of which container has the fresher starter, so you can clean out the old one when building the next gen. I did this for more than 3 years with no infections of any kind.

Besides the initial gluten relaxing that mrwienerdog wrote about, another important effect is the absorption of liquid.

It takes time for the flour to fully hydrate with the available liquid. Before it does the dough will be more sticky and there will be less gluten available for kneading. Giving the recently incorporated dough a short rest allows the flour to pull in all the liquid it can and become much easier to work with.

A similar effect can be seen with pastry doughs. Even though often the goal is to minimize gluten development, a short rest gives the flour time to hydrate and can turn pastry dough from difficult to work and crumbly to soft and smooth.