As you said, there is a large amount of variation. As such, it helps to list a specific college, and even better major within that college. Without this information, it is difficult to give a generalized answer.
For example, ~40% of students in the US who start at a 4-year college do not graduate within 6 years. Those who do not graduate are more likely to have poor grades than the overall student body. If you remove the poor performing students from the average, then the average grade of the class goes up. from freshman to senior year. However, the portion of students who do not graduate varies wildly between colleges. At many of the highly selective colleges that are emphasized on this site >95% graduate. While at some less selective colleges <20% graduate. The reasons for the GPA increase will vary depending at >95% graduation rate college and a <20% graduation rate college.
This post was on the engineering forum, so I’m guessing you are referring to engineering. Engineering has a similar drop out effect, which again differs by college. For example, the Duke study at http://public.econ.duke.edu/~psarcidi/grades_4.0.pdf found that roughly 1/3 of Duke students who said they intended to major in STEM majored in humanities/social science instead, and those 1/3 tended to be weaker students (being female was also a strong predictor that remained after controlling for student quality). Removing the 1/3 who are often weaker students was one of the key contributing factors to the average GPA increase, so STEM saw a larger GPA increase than non-STEM. The average GPA increase from freshman to senior year was ~0.4 for STEM and ~0.1 for non-STEM.
I could also list several colleges that have different trends for different reasons. It’s difficult to generalize.