When using the enchantment table on a vanilla 1.8 server in a test of 200 books, 48 iron pickaxes, 15 stone swords & 37 stone shovels over a range of different levels, 96% of enchantment where a single enchantment.

From what I understand this is extremely unlikely to statistically happen so I figure it has to be a bug of some sort.

So far only experienced this in a server environment, one running command blocks in the background and another running 100% vanilla (no commands).