In agriculture, a bumper crop is a crop that has yielded an unusually productive harvest.
A bumper crop can also be a source of problems, such as when there is insufficient storage space (barns, grain bins, etc.) for an overlarge crop. The word "bumper" has a second definition meaning "something unusually large," which is where this term comes from.
Related meanings or puns
The term "bumper crop" has also been used to refer to a similar large result in other activities, or as a pun such as with a group of automobiles (for their front/rear bumpers).
References
Bumper crop Wikipedia(Text) CC BY-SA