How likely is it that the firms can reach a cooperative arrangement and agree to maintain high prices? Below is a partial list of factors which facilitate collusion.

Fewer firms: Fewer decision makers that must agree. It's like deciding what to get on a pizza.. the more people involved the more likely someone is to want Pineapple.
Lax enforcement of anti-trust law: Just like you can tell where there are fewer patrols on a stretch of highway by the average speed you can partially judge anti-trust enforcement efforts by similarity of pricing.
Similar production costs: If one firm has lower production costs than the others it may wish to take advantage of this to increase market share by pricing lower than the others can afford.