Not so uncommon for these puddle-jumper connector-airlines to do. Same kind of thing happens sometimes IF someone checks in way too early for a flight -- example, checking in early or mid morning for a late afternoon or evening flight.

However, I doubt that the cargo door wasn't closed. The stuff is probably still at SLC if not already being routed to PDX by the time this post is read by OP.

Always a good idea to put an itinerary and contact address inside and also outside of all luggage and gear including carry-on, NOT necessarily your home address as you want your stuff to be delivered WHERE you ARE going to BE, not where you came from.

Good Luck. Hopefully your stuff will find its way to you tomorrow or the next day if it's not already been delivered to you.