Why did "fromes heart.. trembled back to safety"?

Frome's heart trembles back to safety when he sees Mattie pull away from Eady's embrace.

He saw Eady, after a moment, jump from the cutter and go toward the girl with the reins over one arm. The other he tried to slip through hers; but she eluded him nimbly, and Frome’s heart, which had swung out over a black void, trembled back to safety. A moment later he heard the jingle of departing sleigh bells and discerned a figure advancing alone toward the empty expanse of snow before the church.