Why is expired frozen chicken safe to eat? It seems to be a widely held belief that that freezing chicken, and then defrosting, cooking, and eating it much later (well past the expiration date) is a perfectly fine thing to do.

What makes it safe? And are there any food safety hazards to watch out for when doing this?
It's safe because freezing greatly slows (if not completely arrests) the growth of the bacteria that would otherwise make the meat spoil. It doesn't kill them, it just puts them in 'stasis'. The expiration date is given based on the meat only being refrigerated. If you intend to store the meat past its expiration date, best practice is to freeze the meat ASAP, and to use it within 24 hours of defrosting.

The main hazard is in that defrosting. At no point should the chicken (any part thereof) be at room temperature for more than 2 hours. The safest method is to defrost the meat slowly in the fridge. You can also defrost it under cold running water, providing you keep it under the 2 hour mark.

Of course, you can also cook the meat straight from frozen. There is absolutely nothing wrong with doing this for small cuts like chicken breasts. Slicing and cooking a partially defrosted chicken breast is perfectly safe, especially as it often easier to make really thin strips with semi-solid meat. You just have to be extra careful that the meat is at a safe temperature (170°F/75°C).