I know this happens if the battery swells up, but I do not see a spicy pillow in there. That battery is flush with the Digital Crown reader, and it only pops out if I shift the screen away from the Digital Crown (towards the connector). It’s an S1 circa mid-2017, at one point I replaced the screen on it (myself, not got a refurb watch from Apple) which is probably related. It’s also no longer my primary watch, it’s a sleep tracker and a backup in case I forget to charge up my S6 overnight. Lucky bonus, if I do need to replace the battery, the Force Touch sensor is still good so I can reuse that.
at least it'll be easy to change the battery now since it opened up for you. :)
I can close it fine (I have to force it open, it’s just way too easy to) so it’s not of major concern rn (at least, it can wait until after Christmas when I’m not broke while still using it normally). At least it did open up before it became impossible to close up without crushing the battery, so I have a bit of a warning signal.