Alpha Leaves are a broken version of leaves. Nobody knows exactly why they exist. A possible explanation is that in beta 1.2, when birch and spruce trees were added, all existing leaves placed as blocks were randomly converted into either oak leaves or the new birch leaves, as the data value formerly indicating leaf type was changed to indicate leaf decay. Leaves that were not placed, and instead were items in the inventories and chests of players, remained as the old leaf type, which now had its item texture removed but was otherwise the same.
However, leaves were not legitimately obtainable at the time, as shears weren't added until beta 1.7, and experiments with trying to obtain alpha leaves in this way have been inconclusive. But, since Minecraft used to have less inbuilt security measures, and hacked clients could do things they can't now, such as hacking in items with a regular client instead of having to use a backdoor, it is possible that leaf items were hacked in before beta 1.2, and those items were then corrupted when the update happened. And since leaves were not obtainable otherwise, they would have been one of the most common items to hack in. But no one knows for sure.
Like Fake Wooden Slabs, alpha leaves appear in inventory and storage slots as a pink and black checkerboard, which is used by Minecraft to indicate a missing item texture. When held in a player’s or zombie’s hand, they appear as oversized boxes, the size of a placed block, in the hand, with the pink and black error texture on all sides. When placed, they look like normal oak leaves. When put into an item frame, the item frame displays a pink and black checkered cube that takes up almost the entire frame.
Alpha leaves are no longer obtainable, and a way to obtain them in current versions, other than duping existing alpha leaves or spawning them in with commands, is currently unknown, and probably not possible. In 1.13, they will be removed, and all former alpha leaves will be converted to normal oak leaves.