What are some reasons why a rat would lose his tail?

This is definitely a case of degloving.

According to the Rat Health Guide,

"Degloving is a defense mechanism in rats, or other small mammals, that causes the upper layer of skin and tissue on the tail to be sheared or torn away from the bone. It can occur when the rat is grabbed or picked up by the tail, or for example, when the tail is caught in the cage door and the rat attempts to pull away. This injury is not only quite painful to the rat, but leaves the exposed area of the tail open to infection."

While an accident may occur in which a rat's tail get caught, given that more than one of the animals has been degloved, abuse or neglect is likely.

If the injury is not new, and completely healed then there is little you can do at this point besides treat them well and give them a clean, spacious, peaceful home with lots of comfortable things and (if they ask for it) attention.

How are they otherwise?

Hi!

It sounds like your rats have had their tails 'degloved'. This means parts of their tails (the skin and tissue) become detached - this is what happens if someone tries to pick up a rat by their tail or grabs them by their tails. It's extremely painful for rats to have gone through and will probably have left them severely traumatised.

Well done for adopting ratties and I hope everything is going well for them. :)

Holding a rat by the tail can cause it to come off - the pain is unfathomable. It's possible that there was a defect or mom was stressed out. They may have been rescued from a snake or been attacked by another rat even. It could have been the owner's fault for shutting it in a cage door.. Could have even be that they were amputated due to infection at some point.