Of course it is not in a rodeo's best interest to move their dates (due to marketing, familiarity, recognition) but unfortunately, sometimes it's unavoidable.
Unlike other professional sports, dates for particular rodeos are determined by individual rodeo committees. Committees must submit their dates to the sanctioning body and have them confirmed, and available dates may depend upon a variety of local conditions. For example a small rodeo may be moved closer to another local event in order better serve the community that puts on the rodeo.
Also, a sanctioning body (such as the PRCA) may decide to move a rodeo (if acceptable to the local rodeo committee) to better serve the competitors. Rodeo competitors travel thousands of miles each year and moving a particular date may help cut down the travel costs that each competitor incurs.
Locking in a schedule is a fine balancing act that the PRCA does every year and is not as easy as you think it might be. The same can be said of the lower tours of the PBR. Each sanctioning body must look at many factors - travel time, familiar dates (for long-running rodeos), weather, and location in determining how best to serve the rodeo viewing public.