Why are my wall plates bowing outward?
You typically don't use domed head screws to mount the receptacle to the box (they stick out too far).  You can get away with it if the back of the wallplate has a big indentation for those screw heads, but they usually don't.

Those screw heads hold the wallplate away from the surface of the receptacle, so there is a gap between the wallplate and the center of the receptacle.  If you then tighten the wallplate attachment screw to pull the center tight, the wallplate will warp like that.

Options:


Replace the receptacle mounting screws with flathead screws.
Snug the attachment screw just enough to hold the wallplate on.  Don't tighten it until the wallplate bottoms out against the receptacle.


Additional thoughts:


The wallplates were fine for a long time and suddenly bent.

The wallplates in the picture look like plastic. If they were strong enough not to immediately crack, they could have distorted locally, like a dimple under the attachment screw and some stretching around the receptacle mounting screws, and been under stress.  The plastic could gradually degrade and weaken over time through stretching, micro-cracks, etc., adjusting itself for the distortion, until the highest stress area across the middle suddenly failed.  

It is the same product installed in the same way at the same time, so failure would be expected to happen in all of them at roughly the same time.  There could have been some other factor  (sunlight, heat, temperature change, whatever), that pushed them all past the final failure point at the same time. That's all speculation, but materials do behave that way, so sudden bending long after installation wouldn't require magic.

Also, consider that the wall plates wouldn't bend that way unless there was some kind of force acting on them in the same way as what I've described here.
The bend looks much greater than the screw head height.

That's partially an optical illusion.  In the picture, the distance between the wall and the wallplate at the middle and at the ends looks like it's in the ballpark of the screw head height, but the difference would be expected to be greater.

In the first picture, look at one of the receptacles, say the top one, and compare where the wallplate comes on the top edge vs. the bottom edge. The difference is only millimeters. That's in the screw head thickness range. 

The bend looks exaggerated because you're seeing the effect of the bend on the entire length of the wallplate, which is longer than the screw distances, and possibly even additional warping because of the distortion of the wallplate. 


Whether or not the problem is the screws is easy to determine, though. The fixes are trivial. Do either one. If the problem repeats, you will know it was something else.