STEM grad students often have much bigger stipends than humanities grad students given that STEM is more well funded. It sounds like the union simply equalized things, although I’m not sure how that would work with DARPA or similar funding agencies, since RA salaries are taken out of grants (maybe just raise overhead to redistribute more funding to humanities, but grants have caps on overhead as well), so maybe just apply it to TA wages (then anyone in STEM would need to RA and avoid being a TA)? Anyways, it doesn’t take a leap to see how stipend equalization at a university would make it less appealing to those who would get less and more appealing to those who would get more.
What school did you go to? In my grad school, there was very little money for humanities students, and they had to compete for TA positions. STEM money wasn't much of a problem, and most grad students could be RAs, and the opposite problem occurred (they were short on TAs).
It is a zero sum problem, unfortunately.