When average investors act in concert (during periods of panic selling or severe FOMO), they certainly can move the market--but that's not manipulation, it's just market forces in action.
All market manipulations take place with the help of the market participants (or the "average investors") otherwise it won't happen. Take a look at altcoins' pump and dumps. They have to have the pumping groups and other sheep that would participate in it otherwise the pump won't happen nor will it give them any profit.
So when we are talking about investors selling or buying together it depends on why they are doing that. If they are making an irrational decision based on lies then it is still considered market manipulation.
For example if they are panic selling just because some stock market got dumped, or if they are panic buying because some celebrity tweeted something about bitcoin then they are both clear examples of market manipulation.