When Orihime healed Ichigo, she might have healed his power to the time where he nearly became a Hollow during Urahara or the Vizards' training. Another point is that Ichigo might have been able to tap into his Hollow powers every time he fights, so his duration of being H'Ichigo stays longer. The last point I had in mind is how he's slowly being changed into the horse of the "King-Horse" theory. Otherwise, there doesn't seem to be a logical theory supporting his sudden power.