Attacks Single Out A Target?

Sometimes however when I walk away from the gaggle of CWK bosses, the preferred mob of choice switches. For example, whenever I use Blast in the early stages of the boss run, I would be hitting Hsalf. But then when I was walking over to check the status on Margana, then walk back to the other three bosses, I ended up hitting Rellik instead. I'm mystified by that o.O

Oh, and Shidoshi is exactly right. Dunno how the above situation works out though, since they apparently spawn at the same time.