I noticed that Hilchos Challah follows Hilchos Bechoros in the back of the Gemarah Bechoros. Why is this where the Ramban placed his Hilchos Challah?
There’s no Gemara Bavli on most of Zeraim, in particular on Challah or on the related Masechta of Terumos. So Ramban can’t put Hilchos Challah on its Masechta.
If I had to guess why it’s in Bechoros, then, it’d be because that’s the next best place for it: the only Masechta in Bavli which deals with gifts to the Kohanim as its primary subject. (While Zevachim and Chullin both deal with Matnos Kehunah as a major subject, only Bechoros among Bavli Masechtos has it as its main subject.)
I would like to supplement DonielF's answer.
Most mesechtos have the Halachos of the Rif written on them, and one of the exceptions is Bechoros. The other exceptions, other than the purely kodshim mesechtos, are Nedarim and Nidda. Hilchos nidda are included in meseches Shevuos, where there is a sugya about nidda. (They are then copied in the back of meseches Nidda in the modern editions.)
When the Ramban came to fill in the missing mesechtos of Nedarim and Bechoros (which, while being a kodshim mesechta, is mostly practical halacha nowadays, at least in rare circumstances), he did similar to the Rif, and included an additional mesechta that wouldn't have its own halahchos. There is a sugya about chalah in the middle of Bechoros, so that was the most appropriate place to put it in.