Obviously the caveat here is that the "market tendencies" aren't defined democractically. It's very much "voting with your dollar" where the voting itself is limited to what options the market provides and the dollar is limited to how much money each market participant has. In that sense monopolization is very much a natural market tendency although most people in favor of "letting the market decide" over government control/intervention would argue that monopolization is bad and actively harmful to a free market.