Ah but difficulty increases, I think diffeqs was much worse than 2nd semester of calc. Or circuit analysis class was much worse than 2nd month of "ohms law" class. But the dropping out stopped after the first month of the filter classes. I'd expect it to smoothly decline as difficulty smoothly increases.
Isn't that what you'd expect? If the 'easy' stuff was too hard why bother continuing once you know you're out of your depth.