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.

Answer this question based on the passage: where did the term bumper crop come from? OK. Let's think. My stream of consciousness:
To answer the above question, the relevant sentence is: The word "bumper" has a second definition meaning "something unusually large", which is where this term comes from. The answer is something unusually large.