exterior wall panels starting to bulge outwards.

[David Banner]

I bought this home 25 years ago with somewhat new exterior wall panels made of fiberboard type. (not the Hardi Plank cement fiber type). The walls have since been painted over about 5 times so they have much protection on them on the exterior side. Only during this winter in past few months have three of the boards begin to bulge outwards. As if they are not nailed down in certain areas. This has never been a problem before in 25 years. They are not even on the same side of home so it's not as if it's an issue just in one spot of home. Two are on opposite sides of home and the third is on the back end of house. I am guessing something different about this winter has made them finally give way such as excess cold temps or somehow moisture getting inside them(but I'm in northern California Sonoma County so excess cold doesn't really even include any real frost to be worried about). I have no leveling issues with home. Floors are level inside and the roof has not changed in any way to be about weight from the topside. So is there anything else someone has found to cause this issue to happen in a few different areas of the the home after such a long time.