It's not a new problem. P.W.Joyce complained around the turn of the last century in the introduction to his Old Irish Music and Songs collection that jigs,reels and hornpipes were being played much faster then than in the mid nineteenth century . Airs he thought were played too slow.

I think it might be to do with the way we learn tunes. In order to play music proficiently at a slower speed musicians increase the speed when practicing so as to get the tune into their fingers .That way they can play it gracefully when they slow down to the right speed .Some people get carried away with the speed and forget to reconnect to the spirit of the tune.