How does a memory card die?

Memory Cards (or SSDs) have something called "write endurance" which limits the number of times you can write to them. A search for this term or "write limit" will bring up a lot of pages which will give you a lot more detail, but basically:

The number of write cycles to any block of flash is limited - and once you've used up your quota for that block - that's it! The disk can become unreliable.

Source

It's usually quite a large number, but I would assume that it could be affected by things like temperature, humidity, shocks etc.

I would work on the assumption that the drive could fail at any time. Keep it backed up and always carry a spare.

Flash has a limited number of write-cycles, measured on the order of 10000 to 100'000 writes. Some cards have controller logic to mark and avoid segments that have gone bad, but flash will ultimately fail. Environment likely had little to do with it (unless "humid" meant "underwater" or "with heavy condensation dripping off everything").