There is no good reason to throw anything at an employee unless it is an occasional compliment. It would have been particularly ironic if an airborne health insurance packet had injured you.
It is possible that your manager was in a huge rush and as he ran by your desk, he wanted to drop off the packet but misfired. At the same time, it is also possible that he is upset with something or someone, including you, and this was his immature way of showing it.
Either way, the best time to deal with this type of behavior is when it occurs. You could use a direct approach and ask what that was all about, but that could be perceived as an attack. A non-threatening way to get at the issue is to ask your manager if something is wrong. That can open the discussion and also cause him to think about what he did.
If the incident continues to remain on your mind, you can still approach your manager and ask the same question. The way that he responds is likely to provide the answer to many questions about him.
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