MOSFET bulk connection to the source
The reasons for NMOS and PMOS are basically the same. So I'll just discuss the NMOS.

As shown in the picture below, the source and the drain terminal are n+ the body is p-type material. Between source and bulk as well as drain and bulk a diode is present which must not be forwared biased. For this reason the voltage at the bulk must not be higher than the smaller voltage of either the source or the drain terminal.

To ensure this, the bulk is either conncted to lowest voltage available (ususally 0V) or the terminal of the transistor with the lower voltage. By definition the source terminal is the one with the lower voltage. Therefore the bulk terminal is connected to the source.

If the drain terminal was connected to the bulk then the bulk-source diode would become forward biased since now the bulk would be at the same voltage as drain which is more positive than the voltage of the source. This is clearly not desired.