Indeed, his Wikipedia page describes him as being prone to attack anyone who didn't take the marathon sufficiently serious:

Semple was a Boston Marathon race co-director. He had an established history dating back to at least 1957 of physically attacking Boston Marathon runners he perceived to be "non-serious" competitors, whether officially entered or running the course unofficially.[2] In a 1968 interview with Sports Illustrated,[2] he remarked "These screwballs! These weirdies!", crying at the ceiling "These MIT boys! These Tufts characters! These Harvard guys! They write me askin' should they put on spiked shoes for the marathon!" According to fellow race official Will Cloney: "He hurls not only his body at them, but also a rather choice array of epithets... Jock's method of attack is apt to vary." In 1957, Semple narrowly escaped arrest for assault after attempting to knock down a marathon competitor who was wearing webbed snorkeler's shoes and a grotesque mask. "The thing that made me so damned mad," Semple said, "was that the guy was runnin' with the good runners." at the 6.5 miles (10.5 km) mark.[2]

(As I understand it, he didn't take issue with women running the marathon unofficially, like Bobbi Gibb, who ran in the same race and finished well before Switzer.)