Not entirely sure how IEM works all in all, but my first guess would be InTrade's ability to resell shares you've already bought at a higher price, or buy back shares you've shorted to lock in gains. Given that it's not a "pick'em and done" market, there will always be a lot more volatility and swings.
I would be unsurprised if there wasn't a _huge_ flood of Romney shorters wanting to buy back the minute the first debate closed, along with a corresponding flood of new Obama shorters -- this is why InTrade reflected the initial bounce so rapidly, as people moved to make money "right now", and of course the price would drift correspondingly as the people on the other side of those trades could effectively pick and choose.
What we're seeing right now price-wise looks to be over-optimism from the Romney camp, as his bounce will surely recede; there are still plenty of people buying and selling both (with roughly a million-and-a-half shares traded each way). I suspect they're still hoping the trend line will regress towards where it was before the clear spike (
http://www.intrade.com/v4/misc/scoreboard/) for Obama, but it really isn't likely to happen.
Again, a great time to short Romney, IMO. There will be another cliff before the end of this as the polls converge and it becomes obvious that a temporary national poll spike doesn't translate into an Electoral College win.