KenV, not chasing the drogue has nothing to do with rotor blade flex. It's an issue of PIO, you're trying to chase the end of a whip instead of watching the hand that controls the whip.
Chasing the drogue results in large control inputs. Large
pitch control inputs load and unload the main rotor. Watch the video. The HAC (helicopter aircraft commander) initially overshoots the drogue. This resulted in rotor downwash pushing the drogue down. As the drogue moves down, the HAC chases the drogue downward. This unloaded the rotors and they flexed upward. You can actually see the rotors flex up in the video.
By itself, this is no big deal, but in aviation, nothing happens by itself. Unloading the rotor does two things: it reduces drag (which increases airspeed) and reduces the rotor downwash. The increased airspeed results in the overshooting getting worse, which means the drogue is well inside the rotor disk, but below it. Reduced downwash means the drogue starts moving back up. Since the drogue is inside and below the rotor disk, the HAC
must chase the drogue up or risk having the drogue move up into the rotor disk. That would be very bad. When the HAC commands nose up, the rotor loads up again and you can actually see the rotor flex down in the video. In this case the blades flexed downward enough to impact the probe tip.
Note that the helo in the video never went into PIO. The HAC
immediately leveled the helo and stopped its climb right at rotor/probe impact and then moved the helo aft and outward, away from the KC-130 tanker.