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 is a character from Mother Goose Nursery Rhymes. Usually pictured as a very large egg, Humpty Dumpty fell off a wall and broke into many pieces. In the rhyme we hear,
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).
Here is one video version where Alice meets Humpty. If you continue watching, you will hear the poem Jabberwocky a fun poem with lots of nonsense words that make sense.