what would be a reason sole legal custody to the father of my child if we currently share both joint legal and physical custody

The odds don't matter! What matters is the facts of the case, what he can prove and any defenses you may have. You admit that the two of you are not getting along, which means that joint legal and physical custody is contraindicated -- joint parenting requires cooperation and respect for, and with, the other parent. You have two choices: see about having a special master appointed, or go back to court to review the custody arrangements and see what you can do. For your additional information, I have a number of blogs on high-conflict matters that you can find at http://www.hartwig-law.com/blog/high-conflict/. I also have numerous posts on Facebook and Google about these kinds of matters. As to the special master, I have blogs and posts on my website, http://www.hartwig-law.com/blog/special-master/, as well as Facebook and Google. I have also published an article in the Utah State Bar Journal, Co-parenting with a Non-cooperative Parent; Potential Alternatives to Reduce Conflict, which you can find at https://www.utahbar.org/wp-content/uploads/2017/05/May_Jun_2017_FINAL.pdf, beginning at page 18. So, you do have options available to you. These are factual issues that we can discuss. If you are interested in pursuing the matter further, I off an initial 1/4 hour consultation without charge. If you want to set an appointment for the initial consultation, feel free to telephone, or email, my office.
 Be proactive, start prepaing now. David R. Hartwig
801-486-1715