Black tank flush issue

Emptied the black tank today and did as I usually do, hooked up the garden hose to the flush connector and turned it on. The tank had drained to near empty when I turned it on. Lo and behold, the water was not going into the tank as it is supposed to, but was apparently spraying out the vent under the bathroom sink. I found this out when I saw water dripping from the belly while I was outside, and when I went inside and looked under the sink, the vent and most everything around it was quite wet. It had no smell so I know it was fresh water.
Anyone know why it did that, and how do I fix it?