This. Despite of some ppl were telling us, that "It's separate games, separate teams, not affected by each other, blah blah blah", but it was happening in the past too - Ruby Sanctum was delayed due to preparations for Starcraft release.
Also. WOD was obviously abandoned half way to it's end, as it also happened both with Cata and MOP. There two possibilities - good and bad:
Good - Blizzard abandon xpack, when they realize, that direction of development is wrong and moving towards this direction can cause more damage, than content drought
Bad - Blizzard abandon xpack, simply due to lack of budget. I.e. if sub numbers are too low and bring too few money - development of current xpack is being abandoned.
Biggest problem - bad variant is more probable, as in Legion Blizzard don't change direction of development and continue towards concept, that have already failed in WOD.