This problem (high CPU usage by mIRC) has been brought up a few times over the past couple of years.

From what I can recall, the only 'solution' that has worked in most of the cases (exceptions being scenarios that have a lot of scripts running and using a lot of global variables, when local variables would've done the job nicely), is to simply reboot the computer and restart mIRC.

I realize that this probably isn't the solution you're wanting, but rather wondering why this is happening, but I'm sorry to say that there doesn't seem to be any rhyme or reason as to when it happens or why.

I've had this happen on a brand new clean installation, so that eliminates the scripts possibility, but a simple restart of my system and everything worked fine.