Here is a good video of the race
Go to 5:50 in. Simpson is running with her normal stride. At 5:56, when Rowbury is about to pass her, Simpson's left arm goes much wider than her normal stride. Simpson's arm clearly causes the contact.
Not sure if it is a DQ worthy offense, but for those of you who are saying Rowbury caused the fall, you are flat out blind
Simpson clearly caused the contact. You can't cause contact from the front - yes - but you can from the side, which is where Jenny was in relation to Rowbury at 5:56 in the video. Her left arm goes out - why? To impede Rowbury.