Random death while mounted?

TheDutchChef

So was playing with someone... Sitting in queue doing in the meanwhile the weekly endeavor...
We heading towards the next boss in grahtwood, untill i went underneath a tree trunk.
And yea... See video... Did she just bumped her head or so that she randomly died there? Cause i have no clue what could have caused this
Youtube link
Link with video proof of what just randomly happened...