They were probably flushed out of their home underground because of the rain. The fact that they are on compost you just put down is a coincidence.

Rodney

In some ant species, the queen ant will produce a small number of winged females who will leave the colony once a year. This usually occurs at the same time as in nearby ant colonies, often after heavy rains. You will see countless sterile ants out of the nest as well during this time.

Grossaroo. I try and leave the ants alone, even the ones in my garden, but they crawl up my legs and bite me so I can't help but hate that they are around. I guess I'll leave them be for now.

Besides I've got other problems.... my garden which I had thought would be avoided by furry critters (it's surrounded by picket fence on two sides and house on the other, and my dog likes to pee near the garden so I thought the smell would keep them away) seems like it's been visited during the last day or so. My pea seedlings which were doing AWESOME after some heavy rains a week ago look like they were chewed on a bit. Though possibly it was bug damage, I would think a critter would take a bigger bite, these are just some holes/gnarly looking leaves.

Could it be the ants eating the compost? I mixed in a tiny amount of compost with potting soil and compost tea on top of the sandy soil outside, and I always see ants crawling on that area. It makes me think the compost isn't ready and they are eating it. Whoopsie!