I asked Scott Lobdell about it, who wrote Excalbur #75, and he explained that the whole thing was his idea and no one told him to do anything about it. He said that a trademark of Excalibur as a series was how fun and chaotic it was, you never knew what quirky thing would happen next and he figured that this played into that spirit. He just wanted to try something new with the character, specifically citing how Brian’s sister, Psylocke, had undergone a dramatic transformation herself and it worked out pretty well for her as a character, so he wanted to try the same thing with Brian, turning him into Britanic (which seemed like later writers made it Britannic). He noted that it was not intended to be a full-time change, just a sort of fun thing to make Brian (and the rest of the team) go through.
He also noted, “I also liked the idea that the barrel-chested stalwart of the group was going to be forced into the role of the kind of manic member for a bit — it felt fitfully undignified and something Brian might have felt mortified about later.”