If you have used some other crafting job with not enough points to have all the abilities up at same time, then it'll automaticly throw the extras of.
Other than that can't think of a reason.
Really annoying when changing jobs with high level difference, wish they'd just let us assign different ability bars to different classes instead of having to muck about with the ui or macros.
If you're on a crafting class that has 12+ ability points to spend and you put 3 actions on your bar and then switch to a crafting class that only has 10 points to spend, the third action you initially put on your bar will be taken off. The same happens with combat classes if you switch to a class that has fewer points than your previous class...it starts pulling abilities off your bar until you're at or below your allowed maximum for the new class. It's not a perfect system but it's better than it was on open beta where switching classes pulled everything
off your bars and you had to reset them either manually or via macro.