Did you leave the motor in gear when you shut it off? There is a recoil lock on that if you leave the throttle advanced. That motor has the shifter in the tiller handle?
I would buy either a marine battery with threaded posts. Or buy the lead clamps that have the threaded posts and use those. How do you have those other cables terminated?
What do the other set of large cables go to. You can't put a ring terminal on the clamp bolt like that. Since they don't sit flat you get a bad connection.