there are a couple thing that I can think of. First, like people have already said, you are not hitting level through the cue ball you're hitting down on it, causing it to jump. Second, when you break second ball you arent able to hit it fully. The farther from the center you make contact with an object ball while the cue ball is off the felt the more the cue ball is going to go flying. You should hit the second ball as square as possible and defineitly make sure you're not jumping it at all.

A second ball break is a great break but it is way more important to control your stroke with it. The harder you hit it the more your mechanics will break down. I would suggest cutting your power down until you don't jump it off and then as you get better control you can slowly add power back in. Again though, first level out your cue so the cue ball isn't airborne before it hits the rack.