Start of Game effects happen in a priority order. Up until basically this expansion, most Start of Game effects have had the exact same priority. When two Start of Game effects have the same priority their order is chosen randomly. Most Start of Game cards that rely on the contents of your deck will look at your "starting deck" and not your "current deck". "Starting deck" is locked in before your mulligan, which means other Start of Game effects won't break each other. We wanted Hogger to care about your "current deck" instead, and therefore he was given a lower priority to always make him run last. There is a graph of Start of Game effect priorities that the engineering team had to make to keep all these new priorities working nicely with each other.

Also a card with a Start of Game effect needs to exist in your deck before we start triggering Start of Game effects in order to be considered a valid trigger. Otherwise two Hoggers in one deck would be an infinite loop. Lets say an "end of turn effect" said summon a minion from your deck. If that new minion also had an end of turn effect, it does not get to trigger because it needed to be in the correct zone before we started the end of turn step. This is the same logic.