question about strange target window behavior

[Kylaia]

Just wondering if anyone else has experienced what happened to me in game earlier today. Was in a raid to kill the Windstalker Rumbler. I'm currently using an unmodified "simple" Target window, set at the time to display target spell effect icons. Had the Rumbler (a x2 Epic mob) targeted directly. Over the course of the fight, the spell icons & the epic flag in the target window started moving lower and lower on my screen, stretching my target window to about double it's height before finally disappearing from the window entirely. The other target window elements (name, health bar, etc.) remained in place, only the flag & spell effects moved. I have no idea why something like this would occur, and it happened during each of the 3 fights it took our raid party to finally kill the Rumbler. Has anyone else seen this and/or have any ideas on how to prevent it happening again? Thanks in advance!

Ky