how does bottom become an ass?what is the reason for this strange event?

Puck enjoys playing silly pranks on humans. Puck thinks it would be funny, and it is funny, to change Bottom's head into that of an "ass" or a donkey head. The character of Bottom is sort of a buffoon and even his name resembles an “ass”.

lMight add that it comes from Shakespears MIdsummers Night Dream

Spelled Shakespeare wrong apologize