Crank removal tool broke. Before I buy another, what did I do wrong?
You shouldn't use the little adapter that you can disassemble from the tool for a square taper crank. Check, but you should find that it has a larger diameter than fits through the square hole in the cranks. It is used for Octalink, ISIS etc cranks with a larger spindle.

What I think happened is you got a little way with the tool, then the adapter bottomed out against the crank itself (not the spindle). You were then effectively trying to pull the tool apart against itself, and succeeded. 

You probably don't need to buy another. You should be able to use the disassembled puller to remove the crank, and the adapter should still work on suitable cranks, just not this crank or other square taper ones.