Why does my jam often "fall up"? I often get my jam out of the refrigerator, and it looks like this:

That is, all the jam has gone to the top, looking as if the jar has been sitting upside down. However, this happens fairly regularly in both my home and work fridge, so I don't think someone is systematically shifting my jam upside-down and then flipping it back before I retrieve it. :P It doesn't happen every time though. The jar tends to be half to three-quarters full when this happens, but I haven't noticed any further pattern (and I always get standard Smuckers strawberry jam).
I found a very scattered discussion with a variety of hypotheses here. Some of the posters suggest some weird bacteria, but this has happened regularly with different jars of jam, so I'm betting it's not that. My hypothesis is that it's some sort of physics-y effect (something about temperature change with the fridge and pressures?). That said, depending on the answer, Physics StackExchange might turn out to be the wrong place to post this...
Any hypotheses? I'm happy to vary my jam habits to test ideas. :D
 A: I could be wrong, but my guess would be it is caused by a change in temperature. 
At a very simple approximation, the air in the top portion of the jar can be modeled using the ideal gas law: 
$$PV=NkT$$
or $PV=nRT$ if you're a chemist. 
By simple inspection, you can see that if the amount of jam stays constant, and the glass doesn't budge, a drop in the temperature (caused by placing the jar back into the fridge) will lead to either a decrease in the pressure of the air in the top part of the jar or a decrease in it's volume. Let's assume that the lid isn't air tight and the pressure in the system is constant. This means the volume of the gas inside will be decreasing. Because cold air is more dense than hot air, the gas will decrease in volume from the bottom (near the jam) up. 
This decrease in volume of the gas creates a sort of vacuum that pulls the jam up to fill the void of the gas. Because the lid is not completely air tight, some of the gas will escape until eventually the last of the air leaves (or some sort of equilibrium is reached) and the jam sits in the top part of the glass. 
So why does it stay there? My guess is this is a characteristic of the jam itself now. Someone else may be able to expand on this further, but the jam will stay near the top now for the same reason that it will stick to the sides without falling down: high surface tension.
