It can be a vicious cycle. Someone raises price for whatever reason. Their competitors see that and think "well, if it works for them, it'll work for us". Their suppliers see the price rise and want a share of it, so raise theirs too. New players entering the market will likely set prices based off competition, even if the competition has actually set inflated prices. Eventually even companies that wouldn't want to raise prices arbitrarily has to because it's now inflation and their costs have risen.

Even without direct colusion, many companies still end up all following each other.