That was in the constituency where I live. We usually get a higher turnout. The weather was horrendous the night before with a lot of local storm damage all across this patch and some flooding on the boundary of this district, but don't know whether that would account for more than a small percentage. It's a safe Labour seat, so maybe some complacency, too.
My mother's constituency too when and she didn't vote this time. She said she didn't want to go out in bad weather and that it really is one of the safest of 'safe' Labour seats, so I think complacency may be part of the answer, but she also said that she's still disgruntled about the MP expenses scandal.