According to a myth, what was the reason for a war between the city-states of Bologna and Modena in 1325?

The War of the Bucket or the War of the Oaken Bucket was fought in 1325, between the rival city-states of Bologna and Modena. It took place in the region of Emilia-Romagna in northern Italy. The war was an episode in the over 300-year-long struggle between Guelphs and Ghibellines. Modena won the Battle of Zappolino (the only battle of the war), and the bucket (or possibly a copy of it) remains in Modena to this day.

A common myth surrounding the War of the Bucket is that it was caused when the Modenese stole a bucket from a Bolognese well. However, this is mostly incorrect, as the bucket was (according to most accurate accounts) taken as a trophy by the Modenese later in the war. The actual reason Bologna declared war on Modena is because Modena took the castle of Monteveglio from Bologna.