I looked at the formula and that Oxygen double bond attached to a carbon with another oxygen bond... yikes!
I have no idea why this is stable. It appears stable, but you wonder about things like UV light, exposure to some cation, etc...
Well, seems like I have reading to do. (Chem engineer here, I feel like its my responsibility, similar to how programmers read open source code)
PLA is just going to break down into lactic acid. Big deal.
If you look into molecular orbital theory, the double bond causes overlap, which makes the bonds unstable. It can more easily jump off and create a new bond. If you want to see an even worse situation, look at Nitrogen triple bonds.
Further, the extra oxygen attached to the carbon makes the bond a bit polar since you have such a heavy electron clump on one side of the carbon.
That said, yeah, PLA is known to not be very stable, and that's part of the reason its biodegradable. It degrades into lactic-acid based large molecules that living beings have no problem on destroying. (But are toxic when you get several grams of it, so beware if you are doing strange chemical experiments with PLA.)