I don't know if it's roots in general but it always happens to me. Had it happen a ton tonight when I doing harbor.
edit* Just had it happen in the tower. Makes using the 2 spell a gamble.
I've had it happen rarely as a FM, one time I had to fight the floor 3 boss about a half dozen times before I figured it out and stopped using roots and stayed super close.
On another note, I finally did a dungeon with a warlock, and we had two other FMs in the party. Soulburn would go out, and bosses would just melt. Awakened Dual Dragons is the best thing ever.