Pun on 亚历山大 (Yàlìshāndà 'Alexander', as in Alexander the Great). Insert one syllable: 压力 ('pressure') + 山 ('mountain') + 大 ('big') = 'pressure as big as a mountain' — and it sounds exactly like the name. Internet-born around 2012, now mainstream.
register
Casual / humorous register. Fine in chat, social media, and friendly conversation. For formal writing use 压力很大 or 压力巨大 instead.
