He was not going to give Dallas another possession. Play it out conservative, see if the offense can get a first down. Then open things up a bit to go for more to see if they can get points.
If Aaron Rodgers or Tom Brady had been the QB instead of Brock Purdy, three play calling would have been different. I can guarantee, despite all the deserved praise heaped on Purdy, Shanahan does not trust him. I expect a similar plan with the Eagles game. Shanahan will see if he can get something early schematically. If Eagles defense is sound, game will become a brawl.