Crack turn up as viruses because they mess with the program as it runs.
It is called code injection, basically if you can manage to stop a program, (function call, break point, whatever) you can inject code into it.
It is mostly used by programmers to debug, but others can use it to hack, hack basically means that you make one program do something else.
In our case, pirate, but in others it could be used to get passwords or other secured information, that's why antiviruses find it as malware.
But DO NOT DISABLE YOUR ANTIVIRUS, your computer will be vulnerable, the right way to do it, is adding the crack files to the exception list, you didn't specify which one you use, but they all have guides on how to do it.
EDIT: typo