... why do they happen??

 

Anti lag, in my case.

 

Usualy extra fuel being burnt in the exhaust, a decat exhaust allows the fuel to get into the exhaust and for the exhaust to get hot...fuel+hot=POP

 

In my case its too much unburnt NOS

 

In my case it was a HKS SSQ VTA dumpvalve I ran for a while..car runs very rich on overrun = fuel in exhaust = pops. Went back to a recirc as I was afraid it also indicated I'd be suffering from bore wash which cant be a desireable thing.
Rgds
C (in a much quieter 22B)

 

Anit- lag, wrc cars do it all the time, makes for faster acceleration after slowing down . Either that or unburnt fuel as in above threads
Treacle

 

So with Antilag is it the Gems unit you have installed