I don't think it's a reaction of any kind that's breaking the plastic. Aren't they using the same type of plastic as in the 48's? This is just a simple matter of a part of the VF-0 design that can't hold up to stress over time. Also, maybe the design or the plastic is so weak that some units are arriving in their owner's hands pre-broken, although people seem to be breaking theirs during use instead.
Anyone ever see those commercials where they test the durability of a matress by compressing its springs to simulate wear over time? Yamato needs to more thoroughly test their products to see how well they hold up over prolonged use. Heck, if they had plucked any Garland or Zero from the assembly line and tried playing with them for a few minutes I'll bet they would've found problems immediately. Of course, maybe they did but decided to go through with distribution anyway? If Yamato had done that we may all have avoided the Garland and Zero problems altogether.
C'mon Yamato, how many years have you guys been at this? On the other hand, their latest Sonsaku Hakufu figure, complete with "life-like, squeezable breasts" seems to be doing quite well. I'll be sure to squeeze these things daily and let you guys know how they hold up...