Because they are due their major exam, C6 (?)
401-415 were delivered 1999-2000, had their first refresh in 2007/08, so 8 years or so after that, the cycle comes around again.
416-424 followed on from the first batch by around a year, so they would logically have followed 401-415 through works, but obviously they are being diverted off to TSGN now. The extra SPT units (470/471) followed these in 2001. It is possible that the work on 421-424 has been brought forward a little, but 416-420 will just have to hang in there a bit longer until released from Scotland.
425-434, 450-461 & 472-478 date from 2003-2005 and had their first major refresh between 2012-2014 (along with 393-396) so are not due another for 2-3 years.
Given recent speculation about ScotRail keeping more 170s than originally planned (34 instead of 21), I'd suggest that 393-396 and 450-461 will be the 16 going off lease in 2018 (if all goes to plan!), destined possibly for Northern. This will tie in with their next major refresh, so it makes sense.
Given 401-415 are getting charging points, etc, this would suggest that they are staying long-term with ScotRail, with possibly 470/471 to follow them for the same work, then 425-434 and 472-478 through 2018/19 - assuming they are to stay.