I have the exact same problem with my Flex 2-15 but not as severe as yourse yet. While I've used it very (VERY) carefully and not let many other people use it, I noticed today a crack in the housing. Luckily this was before I opened it and broke it all the way through, but it came with normal usage.
The laptop was never fallen or hit with anything and as you explain it's been a little tough to open but in my experience not too much. I've also noticed ealy on the screen was flexing relatively much at that location, so my conclusion is that this is a design fault due to which the housing is not strong enough to withstand this amount of flexing which causes the back cover to crack.
I'm going to try and bring it back for repairs as it's failed before end-of-lifetime, but I haven't got any warranty anymore so I'm curious if I'll get refunded. If not I'm probably going to fix it myself, I'll keep you guys up to date.