It’s a rough definition, but astroturfing is usually rooted in goals separate from or even counter to the stated movement. E.g. in your example outside support for a labor movement wouldn’t necessarily be astroturfing if it’s genuinely supporting labor. A fake labor movement sprouted by the companies themselves to take the wind out of the sails of real labor movements would.

Wikipedia has a more specific definition in the first paragraph:

Astroturfing is the practice of hiding the sponsors of a message or organization (e.g., political, advertising, religious, or public relations) to make it appear as though it originates from, and is supported by, grassroots participants. It is a practice intended to give the statements or organizations credibility by withholding information about the source’s financial backers.

I think it’s pretty hard to argue that labor unions can be astroturfing. It’s not like they have a lot of wealthy donors who are secretly trying to push narratives to encourage workers organizing.