I get the feeling it has to do with how wireless charging works. On a wire, a phone can regulate how quickly it takes charge or whether it does at all. I don't think phones are capable of that with wireless charging, which is exclusively how I charged my pixel 5 at night.

So it would get to 100% and stay there for several hours every single night. I didn't realize it was bad at the time.

It could always just be that I was unlucky and got a defective battery to begin with. No way to know for sure.

Abuse or defects or environment. I've, for example, seen one phone which was constantly woken up (technical term in case it sounds odd) because of some event in the wireless signal and that made it use up the battery in a ridiculously short time. It was a combination of the way a network was set up, bad signal quality, and a firmware quirk. Clearly a defect, but hard to say whose. Forcing it to use some mode in the radio via settings circumvented that.