even before roger called his wife abou the contract; don was giving roger the cold shoulder.

what did i miss?

Originally, Don advised Roger to stay with his family and Roger construed that differently, and told his ex that Don counseled him to leave.

Also, Roger was screwing Don's secretary and Don didn't love that.

Then, Roger called Betty.  It was a smart move, getting Betty in on the badgering of Don to sign...but DD hates when Betty knows things and hates that Roger involved her.

Just what I can recall here...

There may be a little more to it than that - in Season One, Roger comes over for dinner, gets drunk, and makes a ham-handed pass at Betty.  Don caught him after Betty had "disengaged" but it was still an awkward moment, and Don correctly surmised what had happened.  While he did exact a form of revenge through the "oyster lunch", I would imagine that Roger's call to Betty brought up memories of this incident.

I could be wrong about this but in Season 2 when Don was in California and Duck was engineering the buyout of Sterling Cooper by the British, I think Don felt let out of that entire process and was taken by surprise by all the moves with Roger cheerleading for the buyout. And if I remember correctly, they all voted when Don was not there.  This too may have caused some friction.  Plus the incident with Betty,  Roger shagging Don's secretary,Jane, Roger telling Jane about Don's marital problems (when he was staying at the hotel).  All of these incidents may have soured Don on Roger.  And if you think about it, Don does not have any friends.  Roger may have been the closet he came to a friend and some of Roger's actions, to Don, may seem like betrayals.

Good points, guys.  Roger definitely hit on Betty in season 1.  Forgot about that.

And I like the idea of Don not liking Roger's money grab from the Brits.  He obviously doesn't love the Brits, and the officiousness they brought.

Then there was also that slip of the tongue Roger's wife made to Betty.  Forgot that too.

Don has many good reasons to be pissed at Roger.  If I were Don, I'd also be pissed.