Why Do My Subs Pop?

Every time my system powers up or powers down my subs make a loud pop. It didn't happen when I first hooked it up. Can anybody please help?

What kind of subs are they?

The subs are fosgate hx2s. I don't think that'll make a difference though since it also does it with my round solobarics.

its probably the amp, sound like it dosent have a delayed turn on

It is you amp if it does it with both of your sets of subs.