What does mean 横行here? 横行的螃蟹

横行 : Walks sideways is nature way for crab. But it is abnormal for human. It means bullying & unreasonable behavior. 

横: crosswise 

行: walking

What does mean 横行here? 横行的螃蟹

 横行 means outrageous,  against all rules, rampant, run amuck.  This is because humans and most animals walk  or move straight. Crabs on the other hand move sideways.  It's just like in English, "crooked snake".

What does mean 横行here? 横行的螃蟹

横行：螃蟹的走路方式是横着走的，常用来比喻 行动蛮横，倚仗势力做坏事

It's a natural way crabs walks, but we usually use it to say a person who act against law and reason. 

For example, 他看起来是个横行霸道的人。 This sentence means that he loos like a bully.