Unfortunately, Sans is in the middle of something when this happens. This means that shortly after Tanyuu calls out to him, he comes charging straight at her full bore - only to basically jump right over her head and nearly knock a tree over in the process.

It takes a few moments for it to be clear why - but becomes all too apparent when one notices that he's just jumped onto the back of some kind of giant spider nightmare that had been slowly winding through the trees in Tanyuu's direction.