Trappers alone or in small groups had only so much carrying capacity in furs and only so much in supplies to carry them through.  Once or twice a season they had to return to a place where they could sell their bales of furs and replenish needed supplies, especially lead and powder.  There were various solutions to the problem.  The most logical were the nearest Trading Post or a Rendezvous where buyers and traders were ready, willing,  and able to buy and trade.  That's where these guys will end up.