>>281262977He used Aushwalen to get all the fragments of his power back, some quincies survives despite the source of their power being ripped out of them.
Somehow the very little remains of Ywach's power consolidated and were beginning to manifest. Considering how Ywach works, he would have granted some living thing nearby his power and then slowly gain power that way. But the only thing nearby was something that's possibly carrying a hollow, causing it to self-destruct since hollows are poison to Quincy.
But that's just my personal head cannon. Who knows what Kubo actually had in mind.