Never happened. He punches and Fujitora blocks. The one that gets pushed was Momonga, not Fujitora. Besides, the point here is, if people as weak as Zoro or Blueno(didn't happen) could press Fujitora despite the power difference, then Kaku clashing with Zoro would mean equally redundant as an argument in the grand scheme of things because weak characters clashing with a strong character is a common occurrence.