That's a massive and dangerous over-simplification of the market. The market is really complex and the factors that play in determining and moving the price are inter-mingled in a very complex way.
Here are a few ones of the top of my head:
- Someone getting liquidated on a big contract and crashing the whole market with him. (see Oil prices a few weeks ago).
- Someone trading exotic/complex derivatives. His trades on the stock will not make sense unless you account for his whole trade/structure that he created. He can/does move the market in unusual ways.
- Someone trading in a certain way because of taxes. His trades will not make sense if the tax rate was 0%.
- Someone laundering money through the market by buying the stock somewhere and selling it somewhere else in a derivative market making his net position neutral and trying in the process to move proceeds from one place to another. This is, actually, a big one.
- Someone getting out of position when it would be profitable for him not to. But he has better alpha somewhere else he is going to chase.
Here are a few ones of the top of my head:
- Someone getting liquidated on a big contract and crashing the whole market with him. (see Oil prices a few weeks ago).
- Someone trading exotic/complex derivatives. His trades on the stock will not make sense unless you account for his whole trade/structure that he created. He can/does move the market in unusual ways.
- Someone trading in a certain way because of taxes. His trades will not make sense if the tax rate was 0%.
- Someone laundering money through the market by buying the stock somewhere and selling it somewhere else in a derivative market making his net position neutral and trying in the process to move proceeds from one place to another. This is, actually, a big one.
- Someone getting out of position when it would be profitable for him not to. But he has better alpha somewhere else he is going to chase.