Although everyone says it's just because you're not getting out in time (as the cast bar fills), there's actually quite a bit of lag involved too.

When I was doing the story Titan fight everyone else looked like they were going to get hit by the huge line/knockback attack, but then they wouldn't actually take damage. A couple times I thought I reacted quickly and was well out of it but still got hit.

Obviously it is still possible to avoid things for the most part, or none of us would ever get anything done because with many bosses AoE means instant death. Still, by simply looking at other players compared to yourself you should be able to tell there's significant lag.