PWInsider.com is reporting that the reason The Undertaker is back on Raw this coming week, as well as an earlier return for Brock Lesnar, is dealt specifically with the loss of CM Punk.
The original plan for CM Punk was for him to come out after Wrestlemania 29 and complain about his loss to The Undertaker. This would follow up with him walking out on the company. The final decision to hold him off was because the post-Wrestlemania crowd would have most likely cheered him, which is something WWE did not want. Another reason is that CM Punk was very upset with the wording of his promo, which caused some frustration.
About three weeks ago, CM Punk had a massive argument with the top WWE creative officials and was considering quitting the company. Obviously, they worked out their differences and CM Punk was able to work TV show and Wrestlemania. This was also another reason why he was taken off of live events, along with his body being banged up.
There is no set timetable for his return, but it looks like WWE will hold him off until late Summer, potentially leading up to Summerslam in August.