There’s likely more to the story, but since Tufts doesn’t specifically require physics or X years of science, IMO the issue was not telling the university that they dropped the course. Tufts is not unique in not liking surprises like that. Senior year courses are listed on the application. Decisions are made, in part, on course selection. The application signature is validation that everything in the application is accurate.
Lesson fot the future — advise the college in advance of any plans to drop a class