A recent show on PBS about it said that the way the bow was split from the rest of the sub the torpedoes must have detonated in the sub. They suspected, but didn't prove, that one of the depth charges must have dropped right on top of the sub.
I saw that show. Great underwater videos. There is a German researcher that has said for many years that U-166 was sunk because she never returned to port.
Saw it last night - good show, worth watching - “Nova”, I think, because of the technology used to find and reconstruct what happened to the sub.....