Interesting thought, Nev...

I googled leather jacket infestation - from some of the destruction pictures, it could be a possibility. When I walk across the lawn, it starts to sink in 5-10 cm in places, which is why I thought about critters tunneling underneath. Did you find anything like that when you had the problem?

And, if it is leatherjackets, did you use nematodes? (At least that's a true 'pet safe' solution. )

I went out this morning to see if I could find any grubs or larvae... but we have ca 10 cm of new snow!

I may ask the gentleman from the garden center to stop by for a look - he could probably diagnose the problem.

Thanks for the tip!