One possibility is that the vent was on sufficient vacuum to trap a bird or squirrel perhaps? In such case, the siphon could empty the lake until the vacuum was lost (air sucked in from inlet of siphon).

I guess the advantage of having the siphon outlet at lower elevation than the inlet would be faster draining and discharge away from the dam but things like this can happen when the discharge is below the siphon inlet. Probably won't happen again though if that is what happened.

Hope all is well and Otto gives it a clean bill of health.

It isn't what we don't know that gives us trouble, it's what we know that ain't so - Will Rogers