Yay!! Good one Allira!
So, there's three questions that have to be addressed here; How did Cassy send Maruman to the future, what effect did this have on him, and why Maruman anyway?
The first answer, I believe, lies in the true meaning of the word seliga
. A beastspeech word that Elspeth interprets to mean 'behind', a state she attributes to his fits of madness.
Seliga, very obviously, is the ability to not only travel
the dreamtrails, but physically place a part of oneself in that alternative timeline.
Maruman's state of mind when he returns from his fits of madness in - I think it can be explained by remembering, if he IS seliga, than his trueform is in Cassy's time - all the time. How can this be, since he spends such long periods of time in Elspeth's timeline? The ashlings are the key - he can make himself come through and be seen in Elspeth's time.
Remember how in TKP, Maruman tells Elspeth that she won't be seen unless she wants other people to see her (on the ashlings)? Similar thing with Maruman. His body is contained somewhere in the Beforetime, perhaps at a govanmen institute where they are testing on animals with mental powers - similar to how we know they're testing on the Agyllians in Cassy's time for their mental abilities.
So, when he disappears in his times of madness, it's because his physical form is under such great peril, he needs his mind back with himself to focus on the attack.
The last answer ties straight into this; why Maruman? Because he could travel seliga
. It's not an ability many seem to possess, or want to do! As well as that, Maruman's sense of destiny and Cassy's futuretellings would have driven him. His actions are so often driven by prophecies, I don't see the time he was sent to the future being any different.
Now, the interesting part; what effect did this have on him?
His madness, is the effect it has on him.
Or, his apparent
madness, I believe, comes from travelling from timeline to timeline so frequently that he can no longer tell the differences between times, and it's all a blur. That, along with the strain he's under at the govamen institute.
Evidence of this physical toil can be seen in the very first book - one of the first times we meet Maruman. He has a fit, right in front of Elspeth, and mentally screams "the flies!", then passes out. What on earth is that all about? Why include it at all?
It's giving us a clue. Maruman's really in two places at once. His mind is with Elspeth, his body is with the govamen. At that point, I believe he's being tested/tortured. The physical and mental toll on the poor cat is too much for him, and he has to retreat to his real body to focus on the attack (that's why he appears to pass out).
Poor Maruman :(
(The majority of this theory I copied from my Mysteries of Maruman
in the Archives ;) why reinvent the wheel!)