I don't know about who might be trading as 'anon', but I do know a lot of retail traders use TD, so it is quite possible for one small guy to sell via 7(TD) and another person to buy using the same broker #7. If one person wants to sell 100,000 shares and another wants to buy all he can with the cash in his account, perhaps 23,000 or something, while another person wants 7,000 for example, you get those strange numbers and several trades in a row.

This doesn't mean manipulation and underhand dealings don't happen, just makes them harder to identify.