Could also be if the juice is 'fresh'. Have noticed that with some of my fresh-made juices, if I skimp on steeping time there is a whore of a throat hit, and over time that mellows out as the juice develops. Usually the vendor will say if it's made to order/sent fresh/needs a set amount of steeping time, so worth having a goggle at their notes and others' reviews if possible.
And another potential is that it just does not work for you. I had one that just wrecked my throat, left me wheezing and choking, and it was the same brand, PG/VG ratio, etc. as others that were fine, and it just refused to not try to kill me; gave it to someone else and they loved it! So there's always a chance that something in the flavour-cocktail hates you.
Sent from my Nexus 4 using Planet of the Vapes mobile app