Question

According to nelly, why was catherine able to behave so well at thrushcross grange before heathcliffs return

Answer 1

The answer is because Edgar is afraid of making her upset
Edgar realizes that  Catherine was really fond of Heathcliff and he does not want to lose his cool in front of Catherine.
But After Heathcliff's return, the interaction between Catherine and Heathcliff made Edgar really angry and he exploded his emotions toward Catherine.

Answer 2

Nelly views herself as superior or equal to Catherine. According to Nelly, Catherine was able to behave well at Thrushcross grange before Heathcliffs return because Edgar is afraid of making her upset and angry. The foliage on the trees is how Catherine compares her love towards Edgar and Heathcliff's as the eternal rocks beneath.