This is more of a nuisance in PVE.
I use the touch pad a lot in PVE to see where I am supposed to go. If you press the touch pad while driving a sparrow and dismount shortly after your guardian holds the ghost out (as if you just pressed the touch pad). Normally when I am getting off the sparrow I am about to fight something. Therefore, I would prefer to always pull my gun out (whichever one was last selected) when getting off a vehicle.
Thoughts? I only noticed this with the sparrow and haven't tested other vehicles.
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If enabling your ghost was your last action before dismounting, then it makes sense for you to continue that action once you've disembarked.