Rotting pumpkins - any idea why?

I've always grown lots of pumpkins (I have several plots so use them as a kind of groundcover for unused space), but this year for the first time I've had big problems with them rotting away. For instance, in my main patch there were about 8 quite reasonably sized orange or yellow pumpkins in late August, but when I came to harvest them last week as the foliage was dying down all I found were 8 patches of yellow slime.
Has this been happening to anybody else this year? Anybody have any explanation? - for example, wet weather, cool weather, pests....?
Or should I just have been more attentive, and brought them in earlier....?

Stewart

Is the plot or garden secure? Could they have been deliberately trodden on by teenagers for a lark since you last saw them?

I'm in Oxford, and, although we've had lots of chilly nights we haven't had a frost yet  -  threatened in the next few days, I think. It's a secure site, if that can be said about any allotment site, so I don't think it was vandals.

Probably a combination of pest damage, and me not lifting them up off the damp soil......

Thanks for the responses    -    Stewart