Very Simple... There is alot more people trying to sell than buying.

Imagine a long line of people waiting to sell ..... and a short line of people trying to buy.

The transactions are only what other investors can see who "left the line with their buy/sell".... we can get a small glimspe of the line by seeing the L2. Only the MMs really know how many people are in line trying to sell.... and buy.

The MM needs to try and match the lines so its 50/50.... its better for them since they make money on the spreads.... just like the gambling houses..... if too many people line up on one side..... the line moves... up or down....