Check the other forums on here for more information, it's not related to just the X1. This is more common than people would probably expect and there doesn't seem to be a fix for any model.
Dealers often refer back to the manual saying "ASS should not be used in stop and go traffic" and a pretty large list of when it shouldn't be used. The software running ASS reads a lot of sensors and if anything doesn't look right it goes into a failsafe (ie neutral) and there is obviously at least one sensor that is over sensitive or not designed well enough for common situations. Good news is there probably isn't anything wrong with the car. Bad news is you never know when it may do this if it ever does again.
There are also some reports of this happening without ASS enabled but it happens a lot more often when it is. I imagine too a recall would be done if/when BMW finds a solution but it's been going on for a few years and there's been no or real information on what is going on.