The diode is there effectively to change the bipolar sine wave of the mains (with a current pulse every 10mS) to a series of single sided pulses, one every 20mS, with a 10 mS dead time between them. I believe this is because the pump armature wouldn't be able to respond at 100Hz but is ok at 50Hz. The same effect could be achieved by using a permanent magnet armature but this would be more expensive than a diode.
Now, if the pump is being driven by a thyristor, rather than a triac, that too will produce pulses of a single polarity. If these happen to be, say, negative pulses and the pump diode is oriented to use positive pulses the diode in the pump will be reverse biassed and block the current. Reversing the pump connections turns the diode round and all is well.
Edit: if your mains frequency is 60Hz rather than 50Hz then read 8.3mS and 16.7mS respectively for 10mS and 20mS above