It’s always during the phrase where the shadow dragon grabs some rocks to throw them. There will be a single huge gap in between circles almost big enough to fit a circle in if not big enough to do so and I’ve had this happen at least 3 times where a huge rock will come crashing down in the middle of this gap. It’s almost obvious because the empty space disrupts the pattern of circles. There is a missing AoE circle somewhere in there and I’ve witnessed this and witnessed it happen to two other people.
Now I got my achievement for rock dodger and I don’t mind that it takes some luck but when stuff is un-telegraphed like that it just feels very frustrating that even the game mechanics are against you.