The I2C Specification defines a Start condition as a transition of SDA from a High to a Low state while SCL line is high. A Start condition is always generated by the host and signifies the transition of the bus from an Idle to an Active state. Figure 1 shows wave forms for Start and Stop conditions.
A bus collision can occur on a Start condition if the module samples the SDA line low before asserting it low. This does not conform to the I2C Specification that states no bus collision can occur on a Start.