+1 very annoying issue. My best guess is it has to do with latency between the player and the server. While your enduser gaming device is still showing a vehicule being avialable - and you can click on it - it is already being taken by an other player and the server logs it as no longer available - because of the latency, this info is not available for you, leading to a inconsistancy between the server and what is displayed on your screen. .... because of some missing programming (wild assumption on my side) this does not get resolved automatically, but you need to step back from selecting the vehicule, to see it is no longer available. Voila you are back to respawning as infantry.