In Strange's mind, it's obvious as to why he's obeying Shura's order. This is a situation where he's hilariously out of his depth. Not the fighting part, Strange will happily remind people that he served in the war thank you very much, but the part where his animas (who he's developed a strong attachment towards) has suddenly turned into a murder bunny and he has no idea what the hell to do about that.

When Shura reappears, Strange is still holding Bell to him. The hare is back to her normal size and looks perfectly calm and not at all murdery. Strange looks like he needs a fuckin drink.

As Shura approaches, he looks up and nods. "I have no idea how the hell she did that," he bluntly responds. "Or why the hell she's back to her normal size."