This is santa bug. When you press the green icon of santa at the timing of when it dies, Then there is a bug that all defense tower will attack the spot where santa died... The same effect of brute's skill called bring it on. Look how people are using this skill to win.
Well known, but I haven't really seen it blatantly used as you experienced. Well, once Santa is gone that won't be a problem and hopefully it's be fixed before the next special unit is out. Thanks for posting the hard evidence.
I had it used against me like that soon after santa came out (when it was first reported on forums), and there's also the second santa bug running around. Hopefully those poor souls afflicted by the stacked placement bug don't get a double or triple whammy with these other bugs :/
