Why do ion concentrations change with different secretion rates in pancreatic juice?
An interesting question! 

I am attaching the picture from Boron &amp; Boulpaep, Medical Physiology, 2nd Ed, p921 here to clarify the details left out in the hand drawn picture. 



The flow of the digestive pancreatic enzymes increase mainly during food consumption. This is brought about by a number of redundant systems. The fasting secretion of pancreatic enzymes depends on the MMC (migrating motor complex). 



Bicarbonate ions are exchanged with chlorine ions in the apical membrane of the acinar cells through a Chloride Bicarbonate Exchanger. This process is driven by:


Secretin - most important humoral factor - Increases secretion
ACh through muscarinic receptors - Increases secretion
GRP (Gastrin Releasing Peptide) - Increases secretion
Substance P - Decreases secretion


Mechanism of action of secretin seems to be through activation of cAMP. However even small amounts of secretin which do not raise the cAMP levels measurably can stimulate bicarbonate secretion. This suggests secretin response may be by 


Unmeasurably small amount of increase in cAMP
Raise in cAMP levels in localised compartments
Activation of alternative second messenger pathways


In case of GRP the second messenger is not known.

The answer to your question is that when secretin is secreted, the Bicarbonate-Chloride transporter is accelerated to achieve the result displayed. This mainly happens when food enters the small intestine so as to neutralize the acid permitting the digestive enzymes to function at the optimal pH.

The Chloride-Bicarobonate Transporter is an exchange transporter which explains why there is a sharp fall in the chloride levels.