Explanation from him choosing 15 from Terez Paylor in the KC Star...
"(No. 15) still has the five in it a little bit,” Mahomes said. “But at the same time, I was actually 15 my freshman year of basketball. So I just went back to that and wanted to make my own number and do a new thing in this next chapter of my life."
Read more here:
http://www.kansascity.com/sports/nfl...#storylink=cpy