There seem to be lots of factors, including building on flood plains, but also removing trees and scrub on higher ground, which would hold back water (and the EU pay farmers to do this!); straightening of rivers higher up, which speeds up water flow; cuts to environment budgets - this is hotly debated now, as obviously the government don't want to admit that this has happened; climate change; and this year, probably, El Nino, which apparently is a whopper.
Ah well, every year, the politicians swear that they are going to spend more money on flood defences, and every year, it turns out that they didn't, and every year, there are floods.