It is a misconception to assume it was the decision of the miners (or the developers). When miners switch to a software update that is not fully compatible with respect to consensus rules, they think they can afford to do so because they expect they will still be able to sell the mining rewards. So people who criticize consensus rule changes should ask themselves whether they made sure they did not buy Bitcoins that came from these miners initially. If they did, they should admit that they actually paid for the consensus changes they criticize...
If miners or anyone else for that matter run a different software that has different consensus rules and mine blocks that are incompatible with the majority then they will be on a separate chain (ie. an altcoin). Someone who is running a bitcoin client doesn't have to do anything because they won't receive such transactions since they will be rejected behind the scene right away for being invalid.
With a large network like Bitcoin, the decision usually happens before there are two incompatible chains. Incompatible changes get activated when a certain activation threshold (with respect to mining power) is reached. Miners commit to these changes (or not), and other users get to decide whether they buy the mining rewards from these miners, thereby supporting the change with their money.