I'm using the Aardvark adapter as an SPI master using mode 2 or 3. The SCK line does something strange before the actual 8 bits are transmitted. What's wrong with my setup?

When idle, the Aardvark I2C/SPI Host Adapter is not connected to the SPI bus and the SPI lines are left in a high impedance state. Just before an SPI write occurs, the Aardvark adapter will attach to the bus and drive the lines as appropriate. If there are no other devices on the SPI bus to actively drive the lines, the voltage on the slave select line can flutter. It is therefore required that the Slave Select line have a pullup resistor to prevent this fluttering. It is also advisable to have a pullup resistor on MOSI and SCK.

For more information, please refer to the Pin Driving subsection of the SPI Signaling Characteristics section of the Aardvark adapter user manual.