The story
From sponge + case, referring to the “Mocking SpongeBob” meme, where this type of capitalization is used to convey mocking.
Etymology from Wiktionary, CC BY-SA 4.0.
spongecase
Today it means: Synonym of alternating caps (“a form of capitalization where capital letters are randomly placed throughout the word(s), used to convey mockery through text messages or in online discussions”). Full definition →