Yeah, it happens when you remove all the strings at one go. The string tension keeps the bridge in the place so next time when you wanna change all the strings at one go instead of changing it one at a time to keep the tension, you should support the bridge in your perferred position while changing the strings. Hope i helped.
As other mentioned, may be a good idea to re-string one string at a time to maintain tension on the bridge.
However, also keep in mind that if you change the gauge of your strings (i.e. new strings are a lighter gauge than older ones), your bridge will bend backwards because the springs are exerting more backward tension on it than your light-gauge strings are, when tuned to band pitch. In that case, you'll need to open up the back plate and VERY GRADUALLY and SLIGHTLY loosen the screws at the back to reduce the tension of the springs. It will take some trial and error to get it just flat with the strings in tune. And it may take a few tries to get it flat with strings in tune; so each time you turn the screws, re-tune your strings.
Another important tip is to stretch your strings by literally pulling on them (not hard enough to break them, but ALMOST as hard, so as to really stretch the shit outta them!). This will prevent them to stretching as you play, bend and use the trem. Not doing this will result in your guitar going out of tune very frequently despite the locks. It leads to a great deal of frustration for some, as a result of which many go back to fixed bridge guitars and never touch floating trems again.