There aren't that many people named Harrison in TOS that I recall. Could it be he is related to the William B. Harrison who was a member of the BEAGLE, the ship captained by RM Merrick in BREAD & CIRCUSES? If he was P.O.'d about his brother being lost on a ship surveying an earth-type world, he might have prime directive issues (like OMNE in the PHOENIX novels, for those of you who go WAY back) and decide to interfere in his own way with Earth/Starfleet.
Hey, it's ridiculous, but everybody's grasping at straws here (when I checked the episode's teaser just now, I was really thinking the character mentioned was named John Harrison & I was going to have a real a-ha moment (not the band.)