In traditional fall start, the fall semester tends to be more heavily enrolled than the spring semester, since a student who graduates one semester early or one semester late will have one extra fall semester compared to the number of spring terms. Having some students start in the spring is an attempt by colleges to balance enrollment. Ranking-conscious colleges may also admit applicants at the lower end of the stat range for the spring to avoid having them count in stats for “first time fall semester frosh” used for ranking purposes.
Presumably, this also means that colleges would prefer if students who study abroad or do co-op or otherwise take time off campus do so in the fall rather than the spring.