open collector digital output
This series diode is most likely intended to protect against reverse polarity.  The external load is most likely powered by its' own power supply, where the negative lead of that external supply is connected to circuit ground on this board.  The positive lead of the supply feeds the high-side connection of the loads.

Those series diodes are intended to protect the loads and driver circuits against damage if that external power supply is connected backwards.  That is: (+) to circuit ground, (-) to the loads.