A Long pulse indicates a pathological excess of Yin and its interplay with Qi.  It could be excess Damp, Blood or Fluids.  Yin can overflow and fill up more than one position.  The body has not recognized its own limits and has exceeded its capacity.  For example, if there is too much Blood, you might feel that the Liver pulse and the Heart pulse are felt as one Long position as the excess Blood stored in the Liver floods up to the Heart.  A Long pulse can result from pathogenic factors also, as the body tries to shift pathology from one location or one level of Qi or Fluids to another.  

Long-term Long pulses eventually flip, as conversion from a Yin state to a Yang state occurs.  The body creates resistance to overflow and a Wiry pulse can sometimes result.  The body is tempering its own tendency to exceed its own capacity.  

A person might be exceeding his or her emotional limits also.  Long-term anger can lead to a Long, Wiry Liver pulse that extends beyond the parameters of the guan position to include the cun position.  Over time, a Long pulse might send its excess to the ditches (the Eight Extraordinary Channels) as the limits of this aspect of the patient are exceeded.  This is why the Eight Extraordinary pulses tend to be Long pulses. A long pulse can become empty or change quality as it moves to another channel.

A long or overflowing pulse can indicate something trapped where it should not be. Long towards the thumb reflects Yin being trapped on the Yang surface, perhaps as Ying is converting to Wei Qi.