They were afraid that he wouldn't come back to testify and they couldn't compel him to return. He's the prosecution's witness and is responsible for blowing the whistle on Swallow and Shurtleff but is also understandable pissed at how he was (and is) treated by the state after 25+ of civil service.

I personally (and I know him) don't think he wouldn't NOT come back but they seemed to think he was a risk to not show up so they arrested him while he was here for his mother's funeral.

He's out now by the way.