People have usually many reasons to sell their bitcoins, but nowadays the number of Bitcoin's holders increased because people tried to make money as the price go up fastly. They sell usually because they need fiat money, or to invest in other crypto currencies or ICO projects.
You have a very good reason for this. Indeed, most selling bitcoin generally requires that the value of bitcoin be replaced by the legal tender of its own country because the value of bitcoin is meaningful to the holder.