Private taxi service is Illegal in Qatar.  What you were doing is wrong.  Also, someone must have been pissed with you and complained, hence you were caught. or you may have parked to let those guys off at a no parking area.

You will know the fine if the car is registered in your name and the traffic police has your cell number. you will get a notification via SMS

Depending on the gravity, there is a fine and Jail term.