Against bombers, it's because I press the attack in too close and end up clipping the bomber, like this:
instead of like this:
Against fighters, its more varied. One case is when my opponent is slowing quickly, and I'm trying to blow him up before I pass him and push it too close in that attempt, like this:
Another is if we are both going for head-on shots, and I'm gambling on my aim being better than his but end up being wrong.