Yes, they are cracks. When acrylic is stressed you get cracking like what you see in the picture. Does the tank have any supports across the top? If not, this may help. What looks like is happening is the pressure of the water pushing out is bowing the acrylic, so some cross supports should stop further damage. If you can not see these cracks anywhere but at the top, you should be ok, if you provide some support soon.
BTW, all tanks bow to some degree it is a normal happening.
HTH,
cyn