I'm guessing here, but for years Councils have done drainage work, then there were cuts. Silt builds up, the volume of water takes longer to flow away and when it gets to the bottom of the steep valley, the drainage to take it away has also not been maintained and the flood plain floods more than it should, for longer.Not really sure what anyone could do when u live in a steep valley with a river running through it!
Not a lot you can do if councils allow houses to be built in flood plains - the clue is in the name.
Nor if theres an unprecindented shed load of rain...all the more reason for Councils to do the best maintenance they can, not the best they think they can afford / get away with
Edit; I'd like to see a comparison of what Councils spent their money on 50yrs ago and what thry spend it on now...I think there's been some substantial core business mission creep