I try to get the money rock as often as possible. The Silver Shovel adds a chance for all 8 things that pop out to be various gems instead of bells. But several times now I have also gotten 2 100 bell coins instead of the chance to get the 8000 bell bag (which I barely ever get anyway).
They pop out in the same order, just with an extra 100 like so: 100, 100, 200, 300, 500, 1k, 2k, 4k (I assume the 4k comes last although I have never gotten this AND the full 8 items).
What would cause this? I initially thought this might have been a negative effect of using the Silver shovel to balance out the gems. But Ben Brocka suggests your Feng Shui may have an affect as well. I haven't heard anything like it before. I have evidence of it happening another time and I know it has happened a third but I have no proof of that.
Best Answer
The money rock in New Leaf has three factors that determine what you get from it.
The amount of yellow feng shui points you have increases the chance of your potential bell drops being higher.
Once you hit the rock the first time, a timer starts which will give you larger and larger drops until either your potential money rock limit that day is reached, or the timer runs out.
When you carry a silver shovel, there is a chance that the money rock drops gems. This does, however, not increase your chance for more bells.
In summary, place more yellow items to the west in your house and be fast while hitting the money rock.
You can block the knockback using holes, bushes or other solid items, which helps, but judging from your picture you've probably already done that to receive that many bells.