If it happened to you, you want an answer that is honest rather than the confident, cruel version the internet hands out. This is the careful one.
Short answer: attachment style never predicts what any one person will do, and it never excuses betrayal; most dismissive avoidants are faithful. What the pattern can sometimes help explain is why, for some people, closeness becoming deep can quietly increase the urge for distance rather than settle it.
The internet loves to tell you a whole type of person always behaves a certain way. Those claims are false, and they are the last thing that helps when you are hurting. Carefully: for someone whose way of coping is to keep independence and hold feeling at bay, a relationship growing close can feel like something to get relief from. That is a long way from causing betrayal, and the choice is always the person's own, but it can be part of how, in some cases, the need for distance takes a harmful shape.
The betrayal is real regardless of what was behind it. Understanding why is for your clarity and for deciding what comes next; it is never a reason to carry responsibility that belongs to someone else. The book marks where explanation ends and excuse would begin.
Separate "why did this happen?" from "what do I do now?" The first has an answer you can understand. The second is yours, and it does not have to wait for the first to be settled.
Go deeper
The Dismissive Avoidant maps the whole pattern from both sides — how the fortress was built, what it protects, what it costs, and where the door is — with 179 real questions answered.