What causes this glass shower door hinge separation

It looks to me that the hinge is secured to the wall bracket with screws that enter from the wall side of the bracket. If this is the case it would be necessary to remove the screws that hold the wall bracket to the wall in order to gain access to tighten these screws that come loose.

You may want to consider the use of a thread locker product on the threads of the screws that are coming loose. This type of product comes as a thick liquid that you apply to the threads before assembly and then it dries/sets up to help bond the screw into the mating part. You should be able to get thread locker from most auto parts stores and well stocked hardware stores. One well known brand is LocTite.

Before applying the thread locker do clean the screw threads because in a shower application like this there could be a soapy deposit on the screws.

It is hard to say what the cause of the hinge screws coming loose would be. A common reason is vibration in the hinge assembly. It is possible that due to the weight of the glass door on the hinge assembly creates a low level of vibration each time the door is open and closed. From your picture the top part of the hinge has a metal to metal tight contact that could probably use some lubrication (the right side in the rotated picture) to reduce the chance of vibration from occurring.

Update: Removing and Reinstalling Door

You really do want to have a helper to hold the door steady as you remove the screws from the corners of the wall plates. The best advice here is to work out some strategy to place some support under the bottom edge of the door that holds it up just at the correct height. This should support the bottom when the door is in a position that is at right angles to the wall. Such support should be at both the bottom corner near the wall and the other bottom corner of the door. It may be necessary craft up something made with wedges, paint stirring sticks and small blocks of wood. Since this is most likely the nominally "closed" position of the door these supports may very well need to work in conjunction with the lower frame or threshold of the door.

When the door is in the supported position it should be possible to access the corner screws of the wall plates. With the door properly supported the wall plate screws should be able to be removed without putting any strain on the partially removed screws. You should aim to remove both hinges so that the whole door can be moved to another position while you work on that upper hinge. Another thing to consider when removing the wall plate screws is that since the door is in the "closed" position you will need two people, one inside the shower and the other on the outside to be able to work on removing the wall screws. One can hold the door while the other removes the screws on their side and then exchange roles.

Once you complete the work of securing the hinge screws from the back side of the wall plates you can reposition the door back on the supports and slide it back into position. Basically reverse the procedure that was used to remove the door to once again getting it secured to the wall.