Spiders

I was lucky enough to be able to work at home on the patio today. First thing in the morning I pulled off the table cover and chucked it in a corner beside the shed. When I finally put my computers away this evening I retrieved the cover and found a pile of spiders right in the middle.

They hadn't been there when I removed the cover this morning, so hadn't hatched from eggs. Had they been carried there by an adult? Had some kind of nest fallen onto the cover?