Imagine saying "The X 'that was Y'd'"
For example, "The calf that was slaughtered."
This is a general theme. The "that was Y'd" part is called an adjectival subclause because it describes (or modifies) the noun - "the calf" in this case. So to make it more obviously look like an adjective, we might imagine (this is not correct English) saying "The <that was slaughtered> calf".
English speakers realised this point and, lazy folk that they were, decided they liked the idea, but they couldn't be bothered with saying "that was" all the time. So that's why in English today we can say "The slaughtered calf".
You'll note of course that the reason we're left with "slaughtered" (the past participle) is that it was originally part of that 'adjectival clause' with the "that" and the past simple tense.