I don't think the top cap tightening had anything to do with the crack. The expander plug body is capable of cracking the steerer if it is badly overtorqued so if you installed it and overdid it that may have been the cause. Or if you installed it properly and the steerer was defective, that's also a possibility. Overtightening the stem clamps would have distorted the steerer inward so that wasn't the cause either. You must replace the fork.
BTW, the topcap bolts on many cartridge bearing headsets do indeed have a torque spec. It's low, typically 4 to 20 inch-pounds, depending on the headset maker.