If it happened to you when everything seemed good, you are probably asking how. This is a careful answer, rather than the confident, cruel version the internet usually gives.
Short answer: attachment style never predicts what any one person will do, and it never excuses betrayal; most fearful avoidants never cheat on anyone. What the pattern can sometimes help explain is why, for some people, a relationship going well can feel less safe rather than more.
This question gets answered badly almost everywhere, with sweeping claims that a whole kind of person always behaves a certain way. Those claims are false, and if you are hurting they are the last thing you need. The careful version: for someone whose system learned that closeness is risky, a relationship becoming deep and safe can feel like exposure, as something with more to lose. That is a long way from causing betrayal, but it can be part of how distance-seeking, in some people, turns into harm.
If it happened to you, the explanation is not an excuse. The choice was theirs and the responsibility stays with them. Understanding why is for your clarity and for deciding what comes next, and the book marks clearly where explanation ends and excuse would begin.
Separate two questions that tend to arrive together: "why did this happen?" and "what do I do now?" The first has an answer you can understand. The second is yours to make, and it does not have to wait for the first to be fully settled.
Go deeper
The Fearful Avoidant maps the whole pattern from both sides — the Two-Alarm System, the Post-Rupture Cycle, a Decoder at the end of every chapter and 122 real questions answered.