I think at the time I put it down to him being a Scot, but am sure that had nothing to do with it.
There may be something in that, if only a few years in the timing of the change in spelling.
I have a Chambers's Twentieth Century Dictionary, 1971 edition, and that only gives
connexion. In the 1983 edition both are given but
connection is first (implying it is used more). Chambers, of course, was a Scottish publisher and, though now part of an international group, some of that tradition survives.
I have an older dictionary, Dyche's (London, 1777), and that too gives only
connexion. So there is a trend with time here, taken at different speeds for different words and different user groups. The 1971 Chambers's contains "
reflection; also (now chiefly in scientific use)
reflexion". I suspect that in 1777 all of these words taken from Latin had yet to become part of the common speech. Few were then still writing in technical material in Latin, but not long before the likes of Isaac Newton were, so these words had only recently been converted into English words.
The title page of Dyche's Dictionary has direct evidence of that process of the spread of learned words into the wider language: "
A new general English Dictionary; peculiarly calculated for the use and improvement of such as are unacquainted with the learned languages. Wherein the difficult words, and technical terms made use of in anatomy, [...], surgery &c. are not only fully explained, but accented on their proper syllables, to prevent a vicious pronunciation; and marked with initial letters, to denote the part of speech to which each word properly belongs." (Original capitalisation ignored.)