You’re most likely just pulling the needles apart as you go. It happens to most new or newer knitters. In fact, I have this problem everytime I use the backwards loop cast-on. So much so, that I usually shorten the number of sts I cast on by 1 or 2 then add them on at the end of the row because I know I’ll have that length of yarn to contend with… And I’ve been knitting for more than 40 years! You’d think I’d have learned by now, but you’d be wrong!
Anyway, just try to keep the tips of your needles as close together as possible while you work down your cast-on sts. If you still end up with a longish piece of yarn between them, as long as it isn’t a couple of inches, don’t worry too much. It’ll most likely work itself out over time, especially when you wash the piece. Or it really is a worry, you can try casting on an extra st just before you get the end of the row and then knit it together with the last cast on st.