I think I've posted my thoughts on this before, but in brief, this seems to me to be a reaction to passengers thinking they can show up at or after the advertised gate close time because "they're only approximate", "we'll go when they make the final call", or "they'll page us when they're about to close the gate". So the airline makes the advertised time earlier, say changing from T-20 to T-30. This works for a little while, because the people who would have shown up at T-15 now see the gate closing ten minutes earlier and adjust to T-25. But before long they realise that the plane's still boarding and departing at the same time, and indeed that boarding is often not started by the "gate closes" time. So they then start arriving 15 minutes later than the advertised time. And then the airline advertises an earlier time, and so on and so forth.

The same principle applies to check-in times. Aer Lingus, for example, now recommends you arrive at the airport 3½ hours before departure for transatlantic and 2½ hours before departure for all other flights. This is essentially absurd.

BA at its home stations has done a very good job in knocking all this on the head. Check in by T-45, enter security by T-35, and be at the gate by T-20. A small number of passengers will have a bad experience with this, but a missed flight will be a very powerful incentive to be on time next time.

corporate-wage-slave has an advantage in knowing where to rely on the information from his wide experience