Facts are very limited because the boater was alone with no witnesses. A man was found submerged under a packraft, trapped in an undercut rock, mid-river. He had a paddle leash attached to his spray skirt pull loop. There is a challenging rapid 500m upriver, and it seems likely that he tipped there and swam until he was pinned on the undercut rock.
The rest of this is conjecture. Based on his YouTube channel, hidasurf might only have had three months experience, liked running steep creeks at shallow water (Class II-III), and was often or always paddling alone. He had a full safety kit (PFD, helmet, drysuit), frequently scouted rapids, and frequently swam.
Given that his body was recovered with his boat, I assume he was trapped with it. Youtube videos of his boat don’t show any perimeter lines, but do show a red leash connecting his paddle to the spray skirt grab loop. We can’t know for sure, but the leash seems most likely to have caused entanglement. I noticed a swim in one of his videos where he holds onto the boat by looping an arm through the deck combing, so that might have played a factor in his fatal swim as well.