i don't think this is a greedy behavior because some of people want to hold it until another folk coming
and some people is a true fans in bitcoin ,
On top of that, we all know that bitcoin's value is volatile which means its market value will extremely go high at any point of time and that what makes them HODL on bitcoins. The more you HODL while its market value rises up, the more profit you will earn when you decide to sell your precious digital coin.
That is a good explanation. It is being strategical rather than being greedy. When there is a strong trend in any kind of market, you hold it till the trend is broken which is not even close to be broken at the moment. So that is pretty ok to hold it at the moment.