It comes from the initial premise that the 144,000 mentioned in a highly symbolic book is literal. Leap frog to a couple of texts with 'little flock' and 'Christ's brothers' being treated well by others, then hey presto! A two class Christianity is born.
A person in the Christian era is given the holy spirit once they accept Christ. Where in the NT is any evidence of a non-anointed Christian? So if a JW believes they have the holy spirit, then couldn't it be said they have been 'anointed' with it?
This is where OTWO's post comes in:
Until the summer of 1935, it was taught that those spirit-begotten and heaven-bound (spiritual Israel) consisted of the 'royal' 144,000 class and the 'great multitude' (the 'tribulation class' who had fallen short of the 'higher calling' or were 'negligent' so would have to be refined by the 'great tribulation' but could serve as Levites in the heavenly temple). It has to be emphasized that nobody knew who would be relegated to the 'great multitude' class until the 'end' when they were judged. That's why it could be said that the identification of the 'great crowd' had remained a mystery. To reiterate, the 'anointed' consisted of the 144,000 and the 'great crowd.'
But there was also the Jonadab class, companions of the 'anointed,' the 'other sheep' class who had an earthly hope. These were not officially 'Jehovah's witnesses' at the time because, and according to the Vindication books (1931, 1932), they were still 'part of Satan's organization' although they didn't sympathize with it and were 'sighing and crying over the detestable things' etc. (and remember that Isa. 43:10 was talking about literal Israel, so in the Christian dispensation the parallel was with spiritual Israel). And of course, these wouldn't have partaken of any emblems at the Memorial.
Just before the famous Convention in 1935, there was a call for all the Jonadabs to attend it. You've read the account in the Proclaimers book and Yearbook about this Convention? So who rose from their seats when Rutherford asked those who hoped to live forever on earth to stand up?
The Jonadabs, of course!
No wonder there was a hush when Rutherford declared, "Behold, the great multitude!"
And so, with that sleight of hand, the secondary heavenly class was no more, the great multitude became synonymous with the earth-bound Jonadabs, and a whole bunch of partakers stopped partaking, leaving only a small 'remnant' of a literal 144,000 'royal' and 'anointed' class eligible to take the emblems.
And it's mixed in with wanting to have fewer chiefs than indians.