I have tried to reproduce it ... but maybe, I didn't understand correctly ... There is one "special feature" of the sequencer and maybe that could lead you back to where you have been.
If we step-sequence a pattern, let's say at pitch "G" lower then middle "C" - just to have an origin ... then later, if we play middle "C" again to start the sequence, the sequence plays starting with aforsaid "G". Maybe in combination with key-priortities you could go there again? I tried, but didn't succeed.
You are sure that you didn't have more modulation going on? Some keyboard-tracking off ... or the modulation track of the sequencer put to some odd function?
At least it was fun to try it ...