It most likely is the strings under the back plate. I've had the same issue, scared the crap out of me when I first did it. Unfortunately I haven't found a work around for it; I've tried tightening the strings on the back as far as they'll go but the action still remains too high and it still looks like the bridge is being ripped out of the body.

Apparently most strats come with only three springs under the back panel and it can support up to five, which most likely would give you the tension you need to support heavier gauge strings.