You must see things as Jack would: he knows that Walt is chased by Jesse in the desert, he understand he is in big trouble but suddenly he says that everything is okay. The first guess would be that Jesse (with or without some help) have reached Walt while he was speaking and he was threatening him, because if police or DEA were on him (physically speaking) he would have to drop the phone without having the time to cancel the order. At this point, Jack has specific coordinates and has his crew ready to for the combat, and considering their experience and their equipment, they can manage to handle almost anything that Walt couldn't. The only case where he would find himself in a bad spot it is if Walt is surrounded by DEA / Police forces, but he can easily tell if that's the case without exposing too much.
Then once he reaches Walt and he finds him arrested, he considers him too precious and dangerous to be left in the hands of a couple of DEA agents poorly equipped. Finally once he got rid of the DEA, he find out about the treasure and he exploits the situation.
Note that the shovel was not taken from Jack's car, but from Walt's.