Left person radical, the side-form of 人. Marks 件 as a person-related concept - historically a person plus a cow representing one unit of livestock or property. Hence the meaning 'item, piece, individual unit, matter.' Same radical anchors 你, 他, 们.
Right side 牛 (cow). The whole graph reads 'person + cow' - a person counting cattle, one head at a time. Cattle were the prototypical countable property in early Chinese society, so 件 came to mean 'piece, item, instance' more generally. Useful in 一件衣服 (one item of clothing), 一件事 (one matter).
