Early Morning Weirdness

Today, I experienced something that I have never seen before. I seated myself in seat C (row 4). I sat there patiently waiting for the plane to fill with passengers. Soon after I sat down, a lady with her newborn asked if she could take the window seat. In case you're thinking that she (or her baby) was the problem, you're wrong. A few minutes later, a gentleman asked me if he could sit in the middle seat. This perplexed me because the plane was approximately 2/3 full. I reluctantly agreed (I like my space). As I stood up to let the gentleman in, I noticed that across the aisle there were two empty seats (aisle and middle). Utterly befuddled as to why this man did not sit in either of those seats, I decided to sit in the available aisle seat (one row behind me, across the aisle). Nor sooner did I switch seats than the man took my original aisle seat! WTF?! Who does that?