The cops need the dogs as probable cause generators.
I moved to a border area in 2018. The first time I went through a CBP checkpoint I watched the agent tap the side of my campervan; when the dog "alerted" (looked where he tapped) they pulled me out of line for an inspection. Totes legit.
They put me on the bench where the handcuffs were hanging. It took ~45mins to toss the camper interior.
Since then I've been through checkpoints countless times with no pullouts or dog "alerts". IMO the most likely explanation is they wanted an excuse to inspect the camper at first contact.