If the cyclist was behind you when the merge occured, then it looks like you did nothing wrong at that time. If there was not enough room for you to pass the cyclist safely in the right lane, then he/she should have been out far enough to ensure that vehicles didn't try to pass. Did you have at least 3 feet between you and the cyclist when you were sharing the lane before the merge? If I don't have this much, at least, while I'm driving, then I won't attempt to pass. I've actually been flipped off by a cyclist looking back at me because the people behind me were honking when I wouldn't pass him.
My guess is the cyclist somehow figured you were trying to cut him/her off deliberately.
Many car drivers don't pay attention to what's coming up in traffic ahead of them, and many cyclists don't either. Then again, there's also those people, using any form of transportation, that are just looking for an excuse to lash out at others.