The imagemagick transitiion is happening in a period of time in testing that is supposed stop transitions. However much of the transition is security related, so it moves forward, and might violate the (present part of freeze) 10 day holding pattern. Note , it should hopefully be a dull transition, but other programs have popped out and off and back on in sid, so check your dist-upgrades, have apt-listbugs installed, and everything should be fine. It is a rush job around a holiday, things can happen.
To view the transition from upstream
https://release.debian.org/transitions/html/auto-imagemagick.html
Background on the situation via planet debian, just the Imagemagick section, you can follow the links to see the security alerts
https://anarc.at/blog/2016-12-30-free-software-activities-november-december-2016/
If a dist-upgrade wants to remove a program, you can check the transition page and /or check
https://forum.siduction.org/index.php?topic=6524.msg53450#msg53450
peace out
Thanks for heads up and links. :)