SE's 'tracking' method for determining who staggered the mob is based solely on who took the last action before it is considered to be staggered. As such, if you time it juuuuust right, you can trick the system into giving incorrect stagger messages. We had our bard 'trigger' red with a threnody as well, but I don't think I took a screenshot of that.
it happens a lot with battle spam, seems even more so if the person in question is 2-3+ boxing. Servers probably getting confused with all the crap coming from same IP address. Or their programmers just suck -- will to bet the later.
It's funny how the same people that say that people complaining about Empy screenshots are just haters, are the same people complaining about these screenshots (which are far more interesting).