Lack of developed study habits that cater to their individual learning style.
Let's be honest. High school is a joke and the material isn't very advanced. Separate from long, tedious homework assignments, it usually just takes some talent to breeze through it. Others who are less talented academically and need to study may use study habits that aren't fit well for them, but still suffice in a high school environment.
Once these people get to the college level, the material is more advanced and talent is insufficient to doing well and mastering the material. Inefficient study habits or improper study habits will cost you a lot more because the stakes are higher and doing well takes more time. The struggle and panic to find, develop, and hone correct study habits will usually result in a poor freshman year performance.
There are definitely others who go crazy with their independence in college and don't study and party all the time, but I tend to observe the above more frequently.
Working too many hours
Starting with a time-consuming schedule freshman year (17+ credits)
Not preparing adequately and consistently for exams
Not studying until Sunday night (god****it people)
Gimmicky schedules and oddly timed classes
Not actively listening in class (if you never feel lost in lecture, you may be doing it wrong)
Not paying attention in class
Inability to changing study methods as material and coursework changes throughout the year (flash cards vs. rereading for understanding, for example, both have their places)
Not REGULARLY using tutors (two days before an exam doesn't count)
Overlooking small facts that will be tested (it's minor, but yeah, it will doom you in courses like bio and psych if you're not careful)
Inadequate grades in high school (like less than a 3.5 at a standard public high school)
Lack of AP courses taken
I just started college 2 weeks ago... I'm already relying on skills/knowledge from the 5 AP course I took in high school....... not surprisingly, the only class I struggle with is the class that I didn't take an AP course for
College is a different beast, pure and simple. I went to a very competitive high school so I felt prepared for undergrad but that doesn't mean that my study skills were not garbage when applied to undergrad material (they were). In high school you see everything every day (for most high schools) and everything moves very slowly and there is a built in self-check mechanism (HW+frequent quizzes + projects) with the end result being that very little has to be done at home if you are active during school. College is all about managing your own time and even if you still have a very strong knowledge base from high school you could manage your time poorly and do very badly on an exam for lack of having those self-check mechanisms and reviews.
I taught at high school at one point - I refused to babysit them, the parents told me it was "my fault" they their kid had missing assignment because I don't remind them everyday what was assigned at the beginning of the unit - or remind them everyday for an incoming test.
I never realized teaching had a subscript of "and you are also a glorified babysitters".