[mkaras]

I2C bus line drivers are designed as what is called "open collector" or "open drain" which means that the driver only has a provision to turn on and pull the signal line to a low level. When the driver goes off it removes itself from the bus line. The pull up resistor takes over at this point and restores the signal to a high level.

Open drain is used for I2C bus lines so that multiple devices can support pulling the bus line low while at the same time not having to worry about fighting another driver that would be trying to force it high.