"Converted" is not the term that is used, the correct term to use is you got assigned on your short leg. What do you mean by "normal"? It happens if the short option is ITM enough. A bit more details would help to get the full picture and thus provide a better answer to your question. By the way, this question belongs in the Options forum. added: I just noticed that it is an SPY butterfly, so assuming that it is a call fly, my guess is that you were assigned yesterday due to the dividend. Which strike price were your short calls?