How does Holmes explain the cut on Douglas's jaw?

From the text:

“Did you ever know him to cut himself in shaving before?”
“Not for a very long time, sir.”
“Suggestive!” said Holmes. “It may, of course, be a mere coincidence, or it may point to some nervousness which would indicate that he had reason to apprehend danger. Had you noticed anything unusual in his conduct, yesterday, Ames?”
“It struck me that he was a little restless and excited, sir.”
“Ha! The attack may not have been entirely unexpected. We do seem to make a little progress, do we not? Perhaps you would rather do the questioning, Mr. Mac?”