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.