During the Taus advance in technology a few years back the Heavy
Hitting Tau King Crab was created.
It was designed for times when Tau would get stuck on Siege Worlds.
The key to victory for tau is to keep moving.
So a smart commander when bogged down in siege warfare they
would use a Manta to drop in King Crab Tanks to take over for HammerHeads
so they can move and shoot instead of being bogged down and getting destroyed
in a siege battle.
So the King Crabs with extra armor and protection with there slower but move
and shoot abilities can destroy the enemy and be able to hold the lines
in a siege why the rest of the army moves and keeps taking more and more
ground.
Thats my thinking behind it.
Fluff up to you, rules up to you.
Here it is.

And wow yes the claws open and close and inside are burst cannons!

not only do the claws close but then they also move up against the front
of the tank to protect it.
Here is the hammerhead on and other options it can have.
Weapons are Magnetized and can switch in and out with a simple click of the
magnet.. nice.

and the back

Thanks for looking.