I think Leon has the most likely answer.
Take a look at the plumbing system diagram at
http://tmphotos.org/modules/xoopsgal...?g2_itemId=176
The onboard water pump pressurizes the system, using water from the fresh water tank. Pressurized water will try to flow backward, OUT the garden hose inlet, but is prevented from doing so by an anti-backflow device that is built into the inlet. In this case, it appears that the anti-backflow device has failed, possibly due to a bit of grit. With any luck, the fix is as simple as putting a hose on the connector and pumping water through it as fast as possible, to wash out the grit. If this doesn't work, the assembly is relatively easy to replace (inconvenient location to work in, though) and not expensive.
To answer your original question, it is not usual for this to happen. But it is most likely not a problem unless some of the dripping water is finding its way back inside the TM. Of course, it also wastes your water, which may be in short supply if you are dry-camping ...
Bill