I initially thought it couldn't be ticks because I didn't see any
BUT
(warning, my own hairbrained logic at work)
Ticks affect them the most in the winter. The baby ticks latch on in late fall, suck the moose dry, then drop off in the beds come spring. The reason the moose are hit so hard is that in the winter moose can't find open water to immerse in to drown the ticks.
Because of that, you find moose in really stupid places/water in the winter looking for water, especially when covered in ticks. Come spring, they not only loose most of the ticks anyways, but they also get access to open water to stand in and drown the rest.