Advancing to morning (when using a bed) when it is dark enough for mobs to spawn will always result in various amounts of zombie "rain" in the morning. The zombies will spawn somewhere in the sky, fall to the ground, receiving no fall damage, and begin to burn. No other mobs have been seen "raining". Discovered in survival (not a fun surprise), and tested in creative. 'Hard' difficulty and 'Far' view range.
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I just had the same problem but, but I didn't sleep it just started happening as the sun came up and actually continued into mid-day, I was working on a village I found and suddenly a flood of zombies comes pouring in and I watch as they rush the villagers.
I figured it was a bug with zombie sieges, but if you did it in creative without a village then that disproves my theory.
Duplicate of MC-14683
This has been reported before. https://mojang.atlassian.net/browse/MC-14683