jié wài shēng zhī

节外生枝 (節外生枝)

a new branch grows out of a knot

one problem leads to another; side issues keep arising

Example:

快要达成协议时, 对方却又节外生枝。
Kuàiyào dáchéng xiéyì shí, duìfāng quèyòu jiéwàishēngzhī.
An agreement was about to be reached when the other side raised new issues (or caused complications