tl;dr- That peak around 1986 looks pretty difficult to explain in terms of interest. After searching a bit, I found a paper that attempts to explain it in terms of teaching capacity. This paper argues that the rapid rise in student interest in the early 1980's flooded Computer Science departments, forcing them to raise entrance standards to avoid being overwhelmed.
I'm not 100% sure if this line of reasoning is entirely solid, but presenting it here as a partial answer. If nothing else, the idea that departments had logistical troubles accommodating a rapid rise in CS students seems to make sense as a contributing factor.
This answer's based on "A History of Capacity Challenges in Computer Science" (2016).
As we can see in the plot, there's a rapid rise in Computer Science majors until the mid 1980's. It seems generally agreeable that this followed from computers being an interesting field of study.
The rapid increase in student demand at the beginning of the cycle is easy to explain. The late 1970s and early 1980s saw the introduction of the personal computer, which brought many more people into contact with computing.
However, that dip's really hard to explain. The cited paper argues:
If one looks closely at the downturn of the 1980s, however, it quickly becomes clear that the reasons for the collapse in student enrollments had nothing at all to do with student interest. Student demand for computer science courses and degrees remained high throughout that period. Students in the mid 1980s did not decide against majoring in computer science but were instead prohibited from doing so by departments that lacked the resources to accommodate them.
I believe that what happened in the 1980s is best described as a capacity collapse in which universities and colleges were simply unable to satisfy the growing level of student demand. Departments tried a number of strategies to increase their teaching capacity, including retraining faculty from other disciplines and hiring adjunct faculty from industry. In the end, however, demand overwhelmed capacity, and colleges and
universities were forced to restrict admission to the computer science major, which gave rise to the subsequent downturn.
Apparently this shortage of teaching capacity led to increasingly difficult entrance requirements:
The collapse and its aftermath
Even though institutions tried many strategies to expand their teaching capacity, they were eventually overrun by the relentless increase in student demand. Although the report from Snowbird 1980 had warned that “limiting or cutting back enrollments would be counterproductive given the societal need manifested in the rising enrollments,” universities and colleges were forced to do just that. Most of those limitations were based on academic performance and were extremely restrictive. At Berkeley in the mid 1980s, for example, only students with a 4.0 GPA were admitted to the major in Electrical Engineering and Computer Science.
From a systems approach, capacity issues do make some sense for explaining a peak like the one here. The rapid inflow of students could be allowed since they were filling available capacity; then, once that capacity was taxed and admissions was allowing too many students into the classroom, they'd have had to cut back on allowing new students in.
Helps explain the difference in interest-vs.-achievement
The mid-1980's peak is the basically one place where Freshman interest is higher than degree completion.
This can make sense in the capacity explanation, because interested students would've been less able to achieve a CS degree due to rising entrance requirements.
However, it's still strange that student interest drops back below obtainment after that. If it really was a capacity issue, then maybe students reasoned "CS is too hard" or "I don't have the grades/background to get into the CS department"?