So he could run into the RDE and the LT, with no receiver to throw the ball to?
Not to mention that the LDE also has a clear path to him as well.
Corner oneself in, they say.
But most importantly, it't not a tendency as steelb likes to suggest.
Watson was simply moving in the direction he thinks that will give the better chance to complete a pass.
That's all there is to it.
Here is a good example of what you are posting in regards to getting a better chance to complete a pass. Here is Watson, "drifting" to the left.