Dang, that's annoying Netix.

I'd say it is a trait of anything with pumpkin in it as it holds so much moisture. Were the muffins getting too brown on top? Maybe you could out them on a lower tray, turn down the heat a bit (say 160/150) and slow bake for longer...? Is it a recipe you've used before?

This is probably too late for you though, sorry...

The combo does sound delicious!