What causes roller marks in painting?
Possibilities:


Low grade paint
cheap roller or dirty roller, that hold paint in crevices (squirt out as paint is applied)
low nap - I always use at least 1/2"
painting style - basically not varying your strokes, painting unevenly, and the biggest one is pressing down on the roller because paint is low.  Not dipping into the paint bin is the cardinal sin.  A lot of times when you hire someone and the paint is part of the package this is often the problem.  As they try to save on paint costs they make every drop last and you run into issues like this.  


Just know that even with the crappiest paint you can get it on a wall without roller marks.  This isn't rocket science and you should be able to rectify the marks if you are inspecting while painting, before the paint has dried.  But do note if you have roller marks and are using a low quality/thin paint it will be much harder to fix them with another coat as the marks will bleed.  At that point you might need a thin coat of primer.