To take notice of this selling is not a bad thing. If it happens more often, we should be alerted however. So far no reason to get nervous about it.
As we investors all know, to cash in some profits now and then is wise to do. Trading involves risk. For a CEO who has a bunch of shares this is not different. He just received a big pile of shares by reaching some of the predefined targets. To cash out part of that seems not unusual to me.
He probably wants to buy some nice gift for his wife (if he has) for Valentine's Day