From what I've been told, this is a serotonin neurotransmitter malfunction. Serotonin is supposed to be released from one neuron, cross the synapse and carry the message to the next neuron. Sometimes, rather than finishing this transmission at the receiving neuron, the serotonin returns to the sending neuron then makes the same crossing with the same message.

Increasing the availability of serotonin and/or other neurotransmitters can help this transfer complete without the re-uptake.