Why in the heck would my dog do this!?

All right so I was given a husky/border collie/austrailian shepherd mix he's an amazing dog, he'd never hurt anything.... Or so I thought, we live on a farm and we've had him for about 4 months now, he's never had a problem with the cattle or horses or anything. The other day I let him out and he attacked my bottle calf for no reason, I couldn't get him off for the life of me and he almost killed my calf! I had no choice, I had to get rid of him, I cannot have him attacking my cattle and other livestock. I could understand a cat or rodent or something, but a calf!? That's not normal is it!?