Q:

Why is the FBI raiding the house of a whistleblower over leaked SAT questions?

How did this case trigger FBI involvement? Wouldn't that be a civil matter?

A:

From the article:

The FBI is investigating alleged computer intrusion and theft against an unidentified “victim corporation” involving “confidential or proprietary information,” including tests, test forms and internal emails, according to a search warrant issued in the case.

The reference to "computer intrusion" is not precise, but may mean that they suspect a violation of some provision of 18 USC 1030, which makes it a federal crime to access a computer without authorization, or to exceed authorized access, under certain circumstances and for certain purposes.  So it's not necessarily just a civil matter.
Specifically, 18 USC 1030(a)(2) provides:

Whoever [...] intentionally accesses a computer without authorization or exceeds authorized access, and thereby obtains [...] information from any protected computer [...] shall be punished as provided in subsection (c) of this section.

Subsection (c) provides for fines and imprisonment for various terms, making a violation of (a)(2) a felony or misdemeanor, depending on circumstances.  
Subsection (e) defines "protected computer" as any computer "which is used in or affecting interstate or foreign commerce or communication".  It seems very likely that the computer in question was used in interstate communication, and that the SAT does affect interstate commerce.
The fact that they got a search warrant means that they were able to satisfy a judge that there was probable cause to believe a crime was committed and the search would yield relevant evidence.  We'd have to read the warrant, or the corresponding application, to know more.

