On an intraday chart, I see buying and selling spikes that coincide with indicators or news throughout an average day.

But my question is, what exactly is happening when I see a quick major buying volume or sell off independent of news? And I stress 'quick' spikes as some of them jump thousands upon thousands of shares within a fraction of a second.

Are they:
1) Independent investors buying/selling huge volume all at once?
2) Stops/Trailing Stops getting automatically triggered?
3) Other forms of automated triggering?
4) Investment firms/Fund Managers buying/selling large amounts of their portfolio all at once?
5) Firms manipulating the outcome of prices by buying/selling in large volume?