Holding tank waste back flow. Why?

[cmedsailor]

Now that I am reading all the answers and considering what to do, I am concluding that indeed the back flow is most probably just the water in the pipe from the toilet to the top of the holding tank. It's quite long (probably 1.5 meters) and it should almost fill the bowl (comparing to a small 0.5lt bottle of water I sometimes use to flash it and fills nearly half of it).
In this case it's seems like I need to replace the joker valve.
I am wondering though what this twist n lock does. It does twist but not sure if it locks.

 

[Stork_III]

Now that I am reading all the answers and considering what to do, I am concluding that indeed the back flow is most probably just the water in the pipe from the toilet to the top of the holding tank. It's quite long (probably 1.5 meters) and it should almost fill the bowl (comparing to a small 0.5lt bottle of water I sometimes use to flash it and fills nearly half of it).
In this case it's seems like I need to replace the joker valve.
I am wondering though what this twist n lock does. It does twist but not sure if it locks.

The twist n lock has an additional cone shaped seal on the end of the plunger which should seal off any back flow from the joker valve. If backflow occurs when handle has been twisted to the lock position, then the plunger seal is faulty.

 

[bobgarrett]

From my experience I'd agree the joker valve is suspect. It may not need replacing, cleaning might be enough, but having got to it why not just replace it anyway.
As for "why the V" diverter, it will be useful if the holding tank gets blocked until you have time to "poke through".