Q: Activation Lock

For why it happened, difficult to say, but all software has bugs in it.

If you can't remember the id that you were using in Settings > iCloud on the iPad, have you tried : If you forgot your Apple ID - Apple Support ?

If you can't find or remember it, and if you were the original purchaser of the iPad (i.e. it wasn’t bought used), you could try going to an Apple Store (make a Genius Bar appointment if you do) or try contacting iTunes Support online (https://www.apple.com/emea/support/itunes/contact.html) and see if they can remove the iPad from the account - they are likely to want to see proof of ownership before they might help e.g. original receipt with the serial number on it