Junk bypass water valves

My guess is that in all that winterizing, you wound up blowing/dumping just a bit of water into the tanks. The tanks being empty, of course, that water went to the lowest point, the drain valves, and there it froze, causing the valve or the pipe to crack. It wouldn't take but a cup or two of water to cause that, and there's surely that much in the sink/tub drains, let alone in the lines.

That's just a guess, of course.