Potentially rolling out a software change that requires testing out of service...
EDIT: literally just saw this on Twitter. Sounds like there was an issue on the run three days ago with a door opening while in motion that'll probably take them out for a few days minimum while software is fixed. Chris Friend has traditionally been a pretty reliable source, he does a ton of analysis of MBTA data.
That's too bad. They were not in service again this morning. Hopefully it doesn't take too long to fix.