Did you check underneath to make sure you didn't have a plastic bag or something similar stuck to the engine? I know it sounds stupid but it happens all the time. More likely though it's probably the degreaser burning off...the timing seems to suggest that possibility.
Like the bumper sticker says: "Plastic Bags Happen".
I had one blowing down the road, crossed my path, and I couldn't do anything to avoid it. It was one of those heavy duty black trash bags. Made a real mess on the headers, smelled for about two weeks, and took over 6 months to burn completely off. I didn't want to try to scrape or wire brush it off as that could have taken off my ceramic coating.