It's my understanding that many Avoidants become so because they had a high maintenance parent (like a borderline, or alcoholic, or just smothering). Because of this they fear being "engulfed" in a relationship.
Often the tip-off that you're dating an Avoidant is when something happens in your life where most people would offer condolences or assistance, the Avoidant goes silent. I've told three of my Avoidant friends or exes about the boyfriend's cancer. Two have said nothing, one has just forwarded me a couple of political items he knew I would enjoy.