It was bad enough that Eric faced Rodolphus, but he found it very challenging to fight the laughter he wanted so desperately to let out at those words: 'you're not important enough to know my business.' Oh, come on. Does he really think saying that scares me? What a bloody twit. Had Eric not thought Rodolphus would have pulled out his wand to show off and kill him, he likely would have opened his mouth again.
And, yet, within the walls of the Ministry he was safe, but the thought of Rodolphus' dislike carrying out of this moment and when he clocked out for the day had yet to make Eric realise he may regret his attitude towards the capable wizard. Eric was not stupid, he just had a tendency to do stupid things (or say them, rather) without thinking about the consequences long-term. In this case, he did not think that what he said now would affect him later.
"Fine, Lestrange. Be about your business," he said, nearly reminding Rodolphus that he spoke to Eric first and all of this started because of it. Eric bit his tongue, his imagination running wild with what he would have liked to do to Rodolphus for being such an asshat.