It is siphon flow to the print head. The piezo head ejecting ink is what ultimately moves the ink. But the capping station can suck ink through the head to during cleanings or when air might have been introduced into the system, like when changing carts. If the hose that provides suction to the capping station has a leak, then the printer may not be able to clear air from the lines and restore siphon flow. There is probably a separate hose to each segment of the capping station, so one for what would have been the K channels and one more for the color channels. A cracked "ink spike" thingy that the dampers are impaled upon could also cause a siphon break, but would affect just that color.