I'm pretty sure it was in protest of a law saying they'd have to check everyone's ID. The BBC, being incredibly biased, obviously won't report this correctly.
The "implied" link between their fines, them rejecting UK connections, and any new laws is very much a PR thing from imgur.
All the breathless online reporting seems to miss just how toothless the law was, and they still failed at following it.
Like I think the new verification laws are an unworkable mess at best, written by people with an idea similar to believing they could "ban one specific species of fish from UK territorial waters" by throwing the odd grenade in, but they're rather unrelated to what imgur actually did.