Once again, I am stealing from Greg Ellifritz.

Many shooters hold their fingers this way when not firing.  They believe that since the finger is not on the trigger, an accidental discharge won’t occur.  In general, they are correct.  But most shooters don’t know that certain situations can cause an involuntary hand clenching.  The tightening of the hand causes a contraction of all of the fingers with a force up to 30 lbs. (the Glock trigger pull weight is around 6 lbs.) and cannot be consciously controlled.

Trigger Finger Positioning, Hand Clenches, and Accidental Discharges.

One of my greatest bouts of luck was having an instructor that taught be very early on about involuntary hand clenching. He used another wording, but the idea that both hands would act in synchronicity without a conscious order.

Read the whole article, figure out what you do with your trigger finger and fix what needs to be fixed.