It doesn't require the airport to be imminently closing or ATC strikes to cause them to dispatch early. It just requires an opportunity to arise.

Low loads and first flight of the day is a prime target. If slots are available (or irrelevant) then why not get away early? In fact, I suspect the dispatchers working the first wave of departures decide which flights to target in advance.

98.4% of the time you can turn up 15 min before departure and be fine. If you like those odds, and don't mind being delayed (chances are, you'd be transferred FOC to the next flight, but don't quote me) then go for it.

I don't think a 30 minute deadline is significantly more onerous than many other airlines. (Look at T5 conformance, for example.)