太公分豬肉 is the first part of a two-part allegorical saying which has 人人有份 as the latter part, often hidden.
The explanation is rather straight-forward. In the old days, after an ancestor-worshipping ceremony, the head of a clan would carve up the roast pig placed on the altar as sacrifice, and would give a portion to each and every male of the clan. So everyone gets a share.
Nowadays, the expression is more often used to describe an award ceremony where every major contestant gets a prize, irrespective of merits.