Frequent comes from Latin frequēns meaning 'crowded' or 'numerous'. The original sense was spatial — a crowded place. The temporal meaning ('happening often') came from the metaphor of events crowding together in time.
From Old French frequent, from Latin frequēns (genitive frequentis) meaning 'crowded, numerous, repeated, regular, frequent'. The deeper etymology is uncertain, but many scholars connect it to Proto-Indo-European *bʰrek- meaning 'to cram, to stuff, to press close together'. The original Latin sense was spatial — a frequēns place was a crowded one, packed with people
Frequent originally described space, not time. Latin frequēns meant 'crowded' — a frequēns forum was a packed marketplace. The leap from 'crowded in space' to 'crowded in time' is what gave us the modern meaning. A frequent visitor is one who crowds your doorstep; frequent buses crowd the timetable.