This just happened to me in De Other Side on my Monk.
After killing mobs on the first platform I attemped to mount out of combat and run to the next area. However the cast to summon a mount would only reach about 1/8th to 1/4 of the cast bar before being interrupted.
I realized quickly that the damage from Stagger is interrupting all cast times. As soon as the 12 seconds of stagger debuff was gone I was able to mount normally.
This also occurred during the Mueh’zala boss fight. After killing a Shattered Visage and attempting to click a totem, I again was interrupted consistently around 1/8th of the cast time because stagger damage would constantly interrupt the cast.