Typically talking, a display protector bubble arises from a spatial cavity between the floor of your cellphone and the sticky facet of the protector. The protector is meant to be flush towards the display, but when one thing will get in the way in which, slightly pocket of air kinds and bubbles up within the sealed house. You know the way if you pull an overturned canoe underwater, you may stick your head up into it to breathe? It is form of like that.
So what may cause this cavity to kind? The commonest perpetrator is a spot of mud in your display. Even when it is small, so long as it interferes with the stickiness of the protector, a bubble can kind. Bubbles can even kind in case you apply the protector improperly, simply sticking it proper to the display slightly than rolling it downward from the highest.