Ted Cruz, his closest challenger in recent polls, challenged him to a one-on-one debate this Saturday, and two Cruz-aligned super-PACs promised to donate $1.5 million to veterans organizations if Trump agreed.
PACs have a responsibility to their donors who, in this case, contributed to help support Cruz's candidacy. Donating to needy veterans is a great thing to do with your own money. It would be a misuse of donor money when that was given for something else. In this case, the pro-Cruz PACs have made a calculation that highlighting Donald's fear of another debate is worth the small risk that he will change his mind and actually show. And if it shames Donald into the debate, it is worth the investment to create an opportunity to contrast style and substance one more time before voting begins in earnest.
My understanding is that non RNC approved debates are not allowed. The PACs know this. There is no way any of the candidates can do this and not get into hot water.