It is quite common for lions to attack a tortoise. This one was quite a bit bigger than their mouths, so they each took turns in trying to crack the shell but could not get through because of the thickness of the shell.
Usually, it would be the younger lions that would try their luck and patience on a smaller tortoise where they can break the shell and then eat the tortoise.
Now and then you will find a full-grown lion, normally the males, that will go after a big tortoise. The chance of succession is about 50/50 but when they do succeed then it is quite a delicacy to them.