Grass & weeds growing from earth produced in compost bin

I have been composting for years, using 3 bins on a rotating basis, but never before untol this year have I had the following problem:
When I have taken the lovely looking earth from the bottom of the heap and used it in pots for various things, in every one, after a week or two, I have loads of new grass shoots coming up and 1 or 2 other weeds too.
Why is this? I thought composting was suppposeod it kill off any weed or grass seeds?
I am not doing anything different from previous years, and never seen this before. Any ideas, tips welcome please, so I can stop it happening again next year.

Thank you.

 

Thanks to both of you. At least I am not alone. Perhaps I shouldn't have watered it occasionally, which might have kept the temperature down. Strange it's never happened before though until this year. We've had cold summers before! Thanks anyway.