Can't tell whether this is one person or several?
Behaviour like this is sometimes called
Many people criticising one person at once, where what is difficult is the volume rather than any individual message.
One person running several accounts so that a single voice appears to be many independent ones.
An organised group arriving from somewhere else to overwhelm a discussion, a vote or a person.
What would tell them apart
If it's several accounts replying, notice whether they read like separate people with their own history and voice, or like one voice copied several times — the second points toward one person running more than one account. If it's several people, notice whether they arrived together from somewhere identifiable, which points toward an organised arrival, or whether each one seems to have found the thread independently, which points toward an uncoordinated pile-on. From inside a single thread, the honest answer is often that you cannot tell, and that is a normal place to be.
Why it might be none of these
Something might simply have travelled. A post reaching a wider audience than you expected brings replies from people who don't know each other and were never organised by anyone — reach on its own is not evidence that anyone sent them.
None of this covers stalking, doxxing, swatting or harassment campaigns. If any of that is closer to what you're facing, go straight to Red lines instead — the advice on this page was not written for it.