TLDR:
1. Strong foreign language skills from overseas missions.
2. It's easier getting them security clearances since they don't use drugs or alcohol.
3. Mormons are overwhelmingly white and come from a cultural background of conservative patriarchal Christian whiteness which comports with the historical and present culture of the FBI.
Which is funny because most Mormons I know dislike the FBI specifically because their actions as of late are directly in opposition to their beliefs.
The FBI goes after folks like Cliven Bundy. Support for separatism is higher is rural regions, including where LDS are. The LDS Church speaks against nationalism specifically but - like everywhere - folks hear what they want to hear.
"goes after" is a funny way to say "enforces federal law"