What leads to the encounter between Catherine and Heathcliff in the beginning of Chapter 21?

Catherine is now sixteen, and on her birthday she asks Nelly to accompany her for a walk to look for some nesting birds. Cathy runs ahead, and is grabbed by Heathcliff on his property; after realizing who the girl is, Heathcliff insists they come back to Wuthering Heights to rest and see his son.