???
There should be springs in there that stop exactly this from happening! How strange!
The springs keep the screws flush with the guard by always pressing down on the pickup; they also keep the pickup in place as far as height is concerned.
As a stop-gap solution, you can use something else between the underside of the pickguard and the pickup's tabs, cut to size depending on how low you want the pickup. My
Yngwie Strat has rubber tubing around the screws, for example, which helps keep the screws and pickups from popping up like you describe once they've been lowered.
Either way, it seems like someone has robbed your springs.
