I don't know the story behind this. On closer inspection it's not a traditional clinker built boat, but a GRP Orkney or Shetland dory.
When I've watched traditional wooden boats being hauled out in Lewis years ago due to fast approaching Atlantic gales, they usually put round fence posts under the keel and hauled it, replacing the posts as the boat travelled up the slipway. That's labour intensive as you'd have 2-3 men either side of the boat to keep it centred and a couple of others replacing posts and someone on the tractor smoking a Golden Virginia roll-up. There's simply not the men to do this type of retrieval now. Never ever seen them use a boat trailer. If they were in a hurry, it would go up on the steel keel guard!
Slipway could have had algae and the tractor lost control. The council usually spray the slipways to reduce marine growth, but who knows.