Plenty of correct answers for the crooked cut
Angles, length of cutter, sharpned, height of rakers, uneven bar rails, different, larger bar gauge than chain gauge...
One that hasn't been mentioned is how you hold the saw...I've seen many people hold the saw (I teach) so that if you were to let go with your right hand the left hand is holding the top handle in a position that has the saw leaning to one side or the other. The left hand holds most of the weight initially so proper position on the handle will lead to a straight cut, given all else is ok.
The classic is left hand right on top on the flat section of the handle, they then bring the saw over to the right side of their body, as they should to avoid the kickback zone...but do not let their left hand slide over to the curve of the handle, so their wrist is cocked over at an extreme angle that gets uncomfortable real quick...they have a death grip and end up straightening their wrist, thus pulling the saw over on an angle...
I get people to experiment holding the saw in different positions on the top handle (before they start it!) to see exactly what effect it has on the angle of the bar into the cut. Once the cut is under way, all you should have to do is guide it, the weight of the saw and the sharpness of the cutters should keep things going once you've set it on its way...