I don't know why you would tell them anything other than the truth, even if it's a touchy subject like not clicking with a faculty member or not enough funding. Programs like to know if their stipend needs to be more competitive or if it was just a personal choice. I declined at a school that had over half their "first choice" picks decline as well, and they wanted feedback about why their program wasn't chosen first. I told them why, and hopefully my feedback will make the program's recruiting more successful next year.