While I agree with post #2 in so far as indicating the end approaching, the phrase does indeed have a sense of "slowing" or running down/out.
Just like a spring clock that "winds down". This is the flavor of the phrase. Things are happening a bit slower, fewer customers are coming in each hour.
The most obscure meaning - and closest to #2's assertion - is nearing the end of an event where a series of things are scheduled to happen. The last several events may happen with the same "speed", but as we get close to the end of the program, there might be a sense lessening of excitement. Or feeling like, "<sigh> the night is almost over". It would be appropriate to say the night is winding down.
The problem is that this sense of slowing, or "loss", is so common towards the end of events, that I guess we are coming to lose the flavor of the expression "winding down", even if everything is powering on at full energy.
I've been to a concert where, right to the very last number, the crowd was going nuts! It would not work to say, just before the last song, that the concert was winding down.
