I think the straight line misconception has to do with the independent suspension



IRS cars have a different "feel" going in a straight line as opposed to solid axel cars, this isn't new information.



Wider tires only make the problem worse due to poor highway maintenance, for starters, makes the car feel like it's tracking wrong, lower profile tires compound the problem.



As the tires are searching to find the bottom of the rut, the sidewall encounters one edge, the outside sidewall will actually grab the edge of the rut and "throw" the car in that direction and the inner sidewall obviously follows suit, the stiffer the sidewall (lower profile) the more violent the outcome will be.



Wider would almost always = more grip/larger contact patch, as mentioned before, assuming the proper alignment and suspension adjustment steps are taken, if not, the contact patch can either remain the same or get SMALLER in some cases.