The Syrian War is wildly unpopular and he's trying to spread responsibility for it over to Congress.
In an incredible turn of events, Obama made the unprecedented move of actually committing himself to running a Syria War by Congress, after his people spent a week insisting that he wouldn't bother to do it, and after refusing to run an earlier Libyan War by Congress.
So what changed Obama's mind?
1. The Syrian War is wildly unpopular and he's trying to spread responsibility for it over to Congress. This way Republicans won't be able to attack him for it once they have voted for it.
2. It buys him delaying time, since he chose to pull back from the brink, especially once the UK dropped out. This gives Cameron more time to get his country into the war.
Obama has been previously described as hesitant. Discussions of Obama's role in Afghanistan described him as constantly looking for various options and unable to make up his mind.
Going to Congress may undermine his authoritarian stance, but it gives him a credible reason for delaying action further without making him seek weak.