Beached Beavers - any ideas?

Recently in Thanet (Kent), there have been increasing sightings of beavers being found entering the sea. I saw one last week in Deal, very distressed. One was rescued a few days ago but sadly was put down (the salt water is toxic too them). There have been three reports of beavers washing up in the sea in this area in the last two weeks, and many more recently. Does anyone have any suggestions/ideas as to why this might be?