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.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.