Board Thread:The Panopticon/@comment-4028641-20151008023030/@comment-6032121-20200106193112

I dunno, the Doctor did half-address it, didn't she? "Doesn't this ever stop?" Don't forget that the last the Doctor saw of Missy, she had refused the Twelfth Doctor's offer to stand with him, and left on her merry way with the Simm Master. It stands to reason for the Doctor to be only slightly disappointed, but not that surprise, to see the Master up to his/her old tricks again.

And as for the actual answer to the question, it's implicitly that whatever the Master found out on Gallifrey threw him into a murderous rage all over again, I think.