It's hard to have a definitive opinion on this specific incident given the lack of both sides of the story; however, neither situation sounds like the individual was dealing with an IRL emergency. If the tank had time to run back to the beginning and sit down, then odds are they were being rude. The odds are also unlikely that the healer just happened to have an emergency pop up the second they hit level 70.
RL happens, but if you have the time to run back to the entrance or manually leave the instance/shut the game off (as opposed to immediately leaving your seat), you likely had a few extra seconds to say "something came up irl, gotta go, sorry".
I don't play very frequently during the week, so my personal experiences may not be indicative of whether this is really a widespread issue or not, but I'm leaning towards 'not'. It's unfortunate when you run into those kinds of people though.