Digital BW, The Print

I run two black and white EZ setups with C88 printers - one with a CIS (BW1), one with cartridges (BW2).  The BW2 has always printed just slightly darker than the BW1, but it was close enough for my purposes.

I have refilled the BW2 carts three times in the last week (I'm printing hundreds of photos to bind up into books for family members).  Printing continued normally after each of the first two refills. But after the third one, BW2 immediately began to print MUCH darker than BW1 - in film terms I'd say probably two and perhaps three stops darker.  After an overnight cooling off period the results remained the same this morning.  So I swapped the BW2 EZ carts with the third C88 (C88 Color) I have that normally uses regular Epson OEM color carts.  After a couple of cleaning cycles and a purge print to get a good nozzle check, that printer now prints just as dark as BW2 did.  

So logic tells me that something went wrong with one or more of the carts during or after the last refill, although the process seemed to go perfectly normally.  But I'm wondering what it could be?  Has anyone else had this experience with the EZ inks in a C88 - a dramatic and instantaneous change in density?