I think this depends on how you define easy-going and the situation. Easygoingness to me tends to depend on how much you tell people Yes, No, and how often you make waves. The less you say no, the more you are perceived as easygoing. The more you say no the less you are perceived as easygoing.
A. Some people are easy-going because they have very weak emotional investments or attachments even to themselves. It's hard to find what pings them, which allows them to be easy-going.
B. Some people are easygoing because they say yes to everyone, never turn anyone down, are never critical (publicly at least). They can blend in with anyone because they don't project any strong identity and allow others to project whatever identity they want to see on them. These tend to be the path of least resistance people.
C. Some people are easygoing because they have problems defending themselves and this makes them perceived as easygoing. This is slightly distinguished from the type B easygoing person because type B people often choose to be that way for whatever reasons. Type C easy-going person often has an inhibition that prevents asserting themselves.
D. Some people are easygoing because they are like a mosquito constantly buzzing about that never lands on anything for long. They are distinguished from Type A easygoing person because they do form emotional investments and attachments but they tend to be ephemeral and brief. They tend to attach easily and detach just as easily. Since they detach just as quickly as they attach, they have a levity and lightheartedness about them. Once the attachment is severed, they turn into Type A easygoing person.
E. The least easygoing of the easygoing people give the appearance of easygoingness. These are the contextual easygoing people. If you don't ever come across them in a certain context, they'll be fine. These people tend to transmogrify into horrific beasts and then as quickly as it came go back to their former state. You would say, this isn't easygoing, but then I must remind you that if everything is good, they're fine. These are the people you think, "I never knew h/she was like that!" I think I fall into this group.
E. Some people are easygoing based on trust and understanding. These people make a choice just like the type B people. I tend to think these people are the true easygoing people because they transitioned from being naive to making active decisions about how to conduct themselves with others and how to measure their level of attachment, worth, and investment. They're not doing things out of apathy, motives, fear, fickleness, or when times are good.
After this you have the tightwads who can't even pretend to be easygoing if they were strung up by their big toe on a 40 story building with dental floss.