Threats. That's all Poseidon seemed to be good at. Sometimes he actually fulfilled those promises. Other times the Erinyes were able to keep him at bay. It pissed her off that Poseidon was threatening Theseus. She didn't give a damn if he was Theseus' father. Loyalty gained through threats wasn't loyalty at all, it was fear.
"Yeah, I get that. I work for Zeus," she said. "But Zeus doesn't threaten me. Whatever the hell Poseidon has you doing, if you get hurt because of that son of a bitch I'm going to rip out his spine and force feed it to him." If Poseidon wanted to make threats, by all means! Nike was ready to fucking tango!