All the reasons I can think of: GCSEs, UKCAT or BMAT, their personal statement( not enough work experience, extra curriculars, plagiarism, don't come across as passionate for medicine), A level predictions, AS levels, not performing well at interview( don't seem passions or just seem incompetent), DBS check. IB, Scottish highers
Although you could do well in all of these and it could just be that all the other candidates were simply better.