Awhile back I shot a M1 garand from the bench. 8 rounds fairly rapid fire and they strung low and to the right at about a 45 degree angle. Group was about 1-1/2" to 2" from 1st to last. What causes a rifle to string the rounds that way?

Many things can cause it.

Usually on a M1 it is because of the expanding gas system and barrel.

When a M1 is "accurized" this is one of the things that is corrected.

A "dead" op rod can also cause larger groups.

Get a good book on accurizing the M1 to match standards.

Also shoot a 16 to 24 round group and see if the stringing gets bigger or stops at some point, with a good group of shots at the end of the string.

I had a M1 Tanker in 30-06 that would shoot an 8" string group with its first 8 shots.

Let it cool and it would repeat.

A "front end job" cured that problem.

Sounds like your position was changing with the recoil of each shot.

Do you shoot right handed?

Yes, I shoot right hand.

most of the time with a right handed shooter shots going low and right come from jerking the trigger. that is very easy to do in rapid fire