Nothing unusual about flights departing close to each other. 881/883 are two hours apart leaving LAX.
251 and 255 HKG-LHR are only 40 minutes apart.
[And, all 3 of BA's HKG-LHR flights depart within 80 minutes. 11:15p, 11:40p and 12:35a.]
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As for the long layover at SFO, it's not unusual for Asian airlines to leave planes in the US for many hours in the afternoon. The flights can't arrive too late, or passengers can't connect to the Mid-west or East Coast. And they can only depart close to midnight, or they arrive back in Asia too early in the morning.