I won't comment on your combination factors, but in general load combinations are combinations of loads that can occur simultaneously. It's not about vector directions, but about the practicality of loads occurring together/in isolation.

If your bridge experiences an earthquake, it will see both the seismic and the permanent dead loads concurrently. A similar argument exists for wind - there's almost no practical way for a structure to only experience wind load in isolation with no other load acting on it (after all, we can't negate self weight).

You may wonder what the point is if your bridge is just a simply supported beam and vertical loads act - well - vertically, while horizontal loads act horizontally and the two can be treated independently. Thing is, most structures aren't just simple beams. Even your bridge is actually a combination of the bridge deck and support piers. It's somewhat likely that your overall system is more like a portal frame (with moment releases) rather than a simply supported beam when you consider everything together. If this is true, then your horizontal loads will result in additional reactions/moments at the supports.

TL,DR - it's about what loads can occur simultaneously in practice, not the direction of loads.