http://www.thresh.net/2017/03/27/threshold-announce-new-vocalist/
Wilson has left the band before (and this will be the second time he's been replaced by Glynn Morgan), but those other departures were apparently his decision, whereas this time he appears to have been asked to leave by the band. He does have other stuff going on with his solo career and Headspace, so maybe it was felt that was affecting his ability to commit to Threshold, but it's definitely a surprise.
Guitarist Pete Morten also recently left, although him leaving does seem to have been his decision in order to focus on his own band. But while line-up changes aren't anything new to Threshold, with two in quick succession, I can't help but wonder whether there might be something else going on.