My observation of “UC disappointment” threads is that one of the more common scenarios is a student with an impressive-looking 4.4 or 4.5 HS weighted GPA overreaches with the UCs (i.e. does not apply to UCSC, UCR, or UCM), not realizing that his/her UC-recalculated weighted-capped GPA is significantly lower, typically in the 3.8-4.1 range. I.e. s/he applies thinking that UCD, UCI, and UCSB are “safeties” when they are actually high match to low reach (see the second column in the table in reply #1).
It is also common in the “UC disappointment” threads for the applicant to be aiming for a major that is more competitive due to a high number of applicants relative to department capacity. Engineering majors and CS are common majors for this, though other majors may also be more competitive, depending on the campus.