Gray Water Tank Overflow???

I was sitting outside SpongePod last night (enjoying some adult beverages) while grand-daughter was showering. I noticed water pouring out underneath the pod, from the area of the gray water tank and dump valve.  I thought the tank had sprung a leak and I ran over and looked underneath, but saw no leak on the bottom of the tank it seemed to be coming from higher up.  I quickly pulled the dump handle (hose was hooked up) and once the dump started the gushing stopped. Inside, there was some water on the floor outside the shower and underneath the bed, but not alot. I accept responsibility for not keeping up with the tank level; but is there some sort of overflow valve or exit for water without pulling the dump handle or did something come loose?? This morning everything seemed fine.  I am perplexed.