Apple has determined that, under very rare circumstances, a crack may form along the rounded edge of the screen in aluminum models of an Apple Watch Series 2 or Series 3. The crack may begin on one side and then may continue around the screen…
Link to article >>>
https://support.apple.com/en-au/screen-replacement-program-a…
I remember an issue with either Series 3 or 4, where the battery would swell and pop out the screen. Sounds like this is possibly the case with the older series as well (cracking due to battery swelling).
It is possible this is caused by putting the battery through temperature extremes, by leaving it on the charger for too long, but still it is an issue.