As previously reported, Christian’s appearance on TNA’s Slammiversary PPV was a part of the deal that the two companies when WWE wanted Ric Flair to appear the Hall of Fame.
Christian was basically told by WWE to coperate and do the appearance because it affected WWE getting Flair for the Four Horsemen induction. Christian was reportedly not happy about making the appearance.
WWE actually gave approval for TNA to advertise Christian on the IMPACT right before Slammiversary. Dixie Carter decided against it, because she wanted to wait and tease the appearance on Twitter as the event got closer in an attempt to create an Internet buzz.