The male bees are the first to appear from the bee hotels. They hang around in the direct environment of the nests waiting for the females to appear. Emerging females are often immediately approached by one or sometimes two or three males that cling to her back on top of each other. The wait of this bee stack is to much to bare for her and the the ball of bees will fall to the ground.
This female Red mason bee (Osmia bicornis syn. Osmia rufa)  fortunately for has only one male on her back.
References1 Nederlands Soortenregister