The reason you're supposed to rehydrate yeast is so it can take in water in an unstressful environment. Dry yeast act like a sponge when they're rehydrating. They can't control their cellular pumps while they're dry, so anything in the rehydration liquid will rush into the cells. So the must rushed into the cells, and the yeast's stored-up nutrients rushed out of the cells, and then the yeast struggled to pump out the excess sugar.
Any gravity over 1.040 or so has a harmful level of osmotic pressure for unhealthy yeast. Anything over 1.100 or so has a harmful level of osmotic pressure for healthy yeast. Osmotic pressure literally squeezes the guts out of yeast, if their cell membrane isn't healthy enough to resist it.
Basically, you killed most of your yeast by dumping straight into the must. The yeast that was left was unhealthy, and has struggled to ferment your must. I'd expect it to continue to limp along for months. If that's not something you want to deal with, get a few packs of yeast, rehydrate them properly, then add those to the seconday. Red Star yeast is like $0.50/pack and you can't really use too much.