The story
From LGBTQ + QWERTY (the world's most common layout used for computer keyboards), mocking the continuous addition of new letters to the initialism so as to make it explicitly inclusive of as many sexualities as possible, in spite of the plus sign (in LGBT+) already implicitly covering and referring to all of them, whether presently known or yet unknown.
Etymology from Wiktionary, CC BY-SA 4.0.