Q: Why did Humpty Dumpty have a great fall?
A: He wanted to make up for a lousy summer!
Explanation: In my neck of the woods, summer has turned to fall.
Humpty Dumpty sat on a wall
Humpty Dumpty had a great fall
It makes sense that if an egg fell off a wall it would break.
Fall is also a season (spring, summer, fall, winter), sometimes known as autumn. If you had a lousy summer, a bad summer, you might want the next season to be better. That is, you could compensate for a bad summer (make up for a lousy summer) by having a really good autumn.
This joke is funny because it plays with the word fall, the verb (to fall) and the season (autumn).