Surely the IP addresses in your logs provide some clues. Without
seeing them, my conjecture is that boiler rooms in countries with
cheap labor are doing it, in a manner similar to the building up
of characters for resale in online games like Everquest or clicking
on ads to artificially pump up click-through rates. The people
actually doing the work probably have little or no understanding
of English -- they're just following a script provided to them.

Perhaps you need to a little bit of programming to present a
different validation question from a small set instead of the
same one each time.