Todays trading, page-14

Possibly an attempt to call the hand on other earlier investors who wish to take money off the table, and have them have to price their shares at a lower price than such a large parcel for them to gain access to the 'cash'. This may also have additional flow on effect of shaking the belief of other less sturdy investors and have them offload, hoping to possibly buy further stock at lower prices than would have occurred 'but for' such a large parcel order taking place in the queue.

Or, could simply be someone feeling they are over exposed to a particular stock, and realised they actually cannot afford the downside risk.

Who knows? Only the seller in question.