I opened a jar of heavy cream pasta sauce (pumpkin, tomato, cream, etc), ate some and stored the rest in the fridge. The next day it seamed to have sealed itself. The button stayed down when I happened to press it. When I opened it the second time it popped loud. Like new.
Why did this happen? Is it safe to eat? The sauce was store bought. Thank you.:)
If you put hot (or even warm) sauce in a jar, seal it, and place the jar into the fridge, as it cools, it (as well as the air in the jar) will contract. The larger the temperature change, the greater the vacuum that will be created.
Most likely, what you are seeing is warm sauce cooling and creating enough vacuum to suck in that button on the lid. If that's the case, it's perfectly safe to eat.
You say you opened the sauce, used some, and placed the rest (immediately??) into the fridge. That's a perfectly safe procedure for saving leftovers, so long as handling of the sauce did not introduce cross-contamination (ex via "double dipping").
To respond to OP's additional detail that the sauce that remained in the jar was unheated before refrigerating, I will just say that I have had this happen at least once or twice too. It's nothing to worry about.
When a partially empty jar cools, the air inside will cool too. As the air cools, it exerts less pressure upward on the lid. Effectively, the air tries to "contract" inside as it gets cooler, and the (normal pressure) air on the outside of the jar pushes down on the "button."
If the temperature difference is enough when cooling, and there's enough air, the button could push down again. It's the same phenomenon that sometimes can happen if you have a room temperature plastic bottle of some liquid (say, juice), and you empty a significant portion of it before refrigerating it. Several times, I've had such a bottle become indented by the air pressure outside pushing in on the bottle (while the air inside the bottle contracts). As noted in comments, if air pressure changed in your area due to a weather change or something, that could also impact the possibility of the "button" resealing.
It would be exceptionally unusual for a "button" depressing like this to indicate anything wrong with the sauce. Instead, what's more worrisome is when the "button" pops up unexpectedly before the jar is unsealed. Bad bacteria and other organisms could produce waste gases that cause a jar to "pop up." In that case, I'd throw the sauce out. But a jar becoming resealed upon refrigeration? That's normal and just happens sometimes.