Didn't have anything to do with property. There were a handful of pages in The Finished Mystery that came out very strongly against patriotism and basically said a true Christian would not enlist in the military. I'm greatly paraphrasing, but that's the import of it. At the time, during WWI, the US had passed the Espionage Act of 1917. This criminialized any attempt to disrupt the recrutment efforts of the military. The government interpreted those passages in Rutherford's book as an attempt to prevent enlistment in the army. They went after Rutherford and some of the other directors for it.
What the society doesn't like to talk about, and you can discover in the trial transcripts, is that Rutherford actually tried to capitulate and willingly censored The Finished Mystery by having those few pages physically removed from copies of the book, and not including it in further printings. However, a good number of them had already been distributed and the government went after them anyway.
They also were never "exonerated" of the charges like the Society likes to claim today. They got the verdirct set aside on appeal over a procedural matter. The war ended shortly thereafter, and since the war ferver that led to the Espionage Act had died down, it wasn't worth the government's time to proceed with a new trial and they dropped the charges.
In hindsight, it probably would have been overturned by the Supreme Court over First Amendment issues if it had made it that far, anyway.