What is happening on my lawn?

I just went for a look at the garden and saw all these little patches of… Soil? Droppings? On the lawn. There’s LOADS of little piles, and they are made up of lots of individual long grains, or longer squiggly strands. Any idea what they are? We’ve been here 3 years and I’ve never seen them before.

Coincidentally there is a big patch of grass on our lawn that has been brownish since late summer, and that seems to be where all these little mounds are (although it could just be the case that they are harder to see in longer, greener grass – but I was wondering if whatever caused this has also affected the grass)