Well, I'm just guessing here. But, if it didn't leak before the cold weather it is a matter of shrinkage and flexibility. The metal in the radiator is going to shrink ever so slightly the colder it gets. Also, the rubber in the hoses gets less compliant the colder it gets (see Challenger disaster). Combine the two, and you might not get proper sealing. Tighten those clamps and see what happens. Unless your radiator or block has cracked and is leaking from there, then your antifreeze is doing its job.
