I'm no chemist or anything, but the sugar is probably at fault.
Honey is a saturated sugar solution. So saturated, in fact, that it tends to crystalize over time.
Mustard is a mix of solids (mustard seeds) with liquids (water, vinegar and others). Its consistency is determined by how much liquid the solids can absorb.
By mixing both together, you add lots of sugar to the mustard. Sugar pulls liquid out of solids by osmosis. The concentration of sugar outside of the solid mustard particles is much higher than inside. To even out the concentration, water is pulled from inside the solid particles and dilutes the mix, making it runny.
You can observe the same effect if you sprincle fresh fruit with sugar. The sugar pulls the juice out of the fruits. In an industrial scale this method is used to produce dry fruit: substances like sucrose, invert sugar, corn syrup and of course salt draw the moisture out of the solid fruit or vegetable and leave a relitively dry product behind. (source)