I also noticed this being an issue on my warlock. However, I have not noticed it on my hunter (like others have noted). I don't think it happens after dismounting, but usually only when I initially summon a pet. Like Xrystal mentioned, entering combat usually makes it pop back up, so I just kinda started chalking it up to the inherent instability of warlock-summoned minions (something Blizzard intentionally built into the warlock pet system).
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