My theory based on the preview (its short) : One day,Steven is going to hang out with the Gems, saying that he was to be involved in the misson but the Gems turn him down, saying that he still doesn't have control over his powers then that's when Uncle Grandpa will appear. Steven tells him whats the promblem and then Uncle Grandpa thinks that the Shield will only appear if Steven is in danger, hence the bazooka pointed at Steven. Let me know about your theories and I'll check it out later

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