Minecraft inventory dragging glitch

I know you checked your shift key, but 1 explanation would be that you pressed shift multiple times resulting in an affect called sticky keys. One way to remove it is press shift repeatedly until your computer has a pop up message. Then you have to press cancel or something.

Sorry if this isn't specific enough.

Items can be moved rapidly from your hotbar to the crafting table quickly by pressing the corresponding number while hovering your cursor over the desired destination slot in the crafting grid.

This is most useful when crafting a lot of something that takes only one ingredient.

1. craft a few of the destination items the "slow" way and use those items to reserve slots for that item in your inventory. At this point your inventory should be totally full, with some of the ingredient in slots 1 and 2 of your hotbar.
2. Arrange for a large amount of the ingredient to be on the ground at your feet. This can be done by breaking a chest full of only the ingredient.
3. open the crafting grid, move your cursor to the desired slot in the grid and press 1 and 2 to move the items into the grid. As you do this, more items will be picked up off the ground to replace them.
4. Shift click the crafted item into your inventory into the slots you reserved for it in step 1.
5. repeat steps 3 and 4 until no more items are on the ground.

Here's a video by jl2579 demonstrating the technique