Lots of things can cause it. Heavy rain can be one cause. Rivers
can also get blocked up making it more difficult for water to get
through. They need to be dredged every so often to prevent that.
Sometimes work can cause rivers to be diverted or blocked, which
can cause them to overflow.
A quick way would be to drop into a plumbing supply store. They
often have tubs on display with the overflow drain hooked up. You
will be able to see where the overflow water goes down the drain.
It can be caused by continual use by heavy vehicles but more
often than not it is caused by water collecting on the tarmac and
when it freezes it expands and forces the tar apart causing cracks
and eventually, holes.
Cloudbursts are sudden rainstorms caused when nimbus
clouds become saturated with water vapor in the atmosphere and
release large quantity's of heavy rain. Flash flooding is often
associated with these downpours.