Ichigo's hollow powers

[anphorus]

I think his soul-chain being eaten away probably activted it. The ball-thingy's seal looked intact when Aizen removed it from Rukia.

[Honinbou]

Hmmz..
Didn't Aizen said that there was only 1 way to get more power then a shinigami can get cuz shinigami powers have limits, so he found out that the only way to get more power then the limit is to become a hollow.
Ichigo is always looking for more power to protect his friends etc,
I think it has nothing to do with alot of his "ancesters" or any of those theories, probably ichigo was the first one who found out how to get to the next level, but he can only do it when he or his friends are in life danger.
sounds pretty logically to me...
so every shinigami can become a hollow once unlocked and knowing..