Some weird goings on in the universe has made the entire universe warped, merging dimensions and causing chaos. The crimson cosmos finds itself merged with the chaos dimension that Shuma-Gorath resides in. Cyttorak doesn't like this presence in his cosmos, and Shuma-Gorath views Cyttorak as yet another entity to feed off of. A battle for supremacy ensues between the two. Because of the merging of home dimensions, both have their full power at their disposal. It's all out, full-power, no holds barred.
Let's use as much comic book "logic" as possible.