In Cambridgeshire, the tradition of pelting the harvest wagon with water was a part of the celebrations known as "Horkey." This practice symbolized the end of the harvest season when it no longer mattered if the produce or people got wet, signifying a time of relaxation and festivity after the hard work of harvesting crops.
For more detailed information, you can visit the reference from Wikipedia on Horkey.
Here’s an example image from a related tradition: