Draco was surprised at first, men did not come to the Manor to meet with him, especially when it was to discuss His business. They came to meet with his Father.
But the rumours about Snape were sweeping - Snape had murdered Dumbledore, he knew that much at least from things caught from the Death Eaters who frequented the Manor. He was trusted far more than he had been before, after that.
It gave Draco even more a reason to hate him.
But he followed when he was beckoned, because saying no was tantamount to saying he didn't care what orders from the Dark Lord he might have brought.