If you are holding 2 shields at once, block with the main hand shield, and switch to another main hand slot, it will automatically start blocking with your off-hand shield. However, if you do the same thing but instead throw the shield on the ground, it will not automatically start blocking with the main hand shield.

Steps to reproduce:

1. Hold a shield in both hands
2. Block. Notice you block with your main hand
3. Switch to another inventory slot. You will begin blocking with your off-hand.
4. Stop blocking and switch back to the original shield.
5. Start blocking again.
6. Throw the shield on the ground. You will not be blocking with your off-hand.