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

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.