hm heres some ideas i have (and one for each god :D)
the guardsmen have become bloodthirsty and ravenous or have located a deamon weapon of khorne which has corrupted them
infected with a plague or about to die and give into nurgle so they can therefore live
slanessh - sexy , drugs ,rock and roll or you could have them under the control of an deamonette ...
and finally tezennch (also the chaos god with the worst name lol)
the psyker seeks more power or knowladge or wants to take over and plots against the commisar for example
my 2 cents
hope this helps you dude
" The extra spears were to make it more killy, I don't give a damn if it looks chaos-y or not. It's made to kill shit." crimson_chin
*picture of a Cultist riding a Trygon*
Whee captoured eet fhour CHAOZ!