So that when year 99999 ends, the Y100K bug can happen, I guess. No idea why having fixed-length 5- (or even 6-) digit strings for years is preferable to normal, variable-length decimal notation we use for numbers: 48, 112, 988, 1122, 2024, etc. The year is a number, treating it as a lexicographically-imbued string only matters for things like ISO 8601 (20240724T vs. 020240724T) but even then, it's already properly ordered: both "99991231T" and "099991231T" sort before "100000101T".
But hey, everybody has their small idiosyncratic pet peeves: I've ran twice into the same person here on HN who just can't stand term "x64" because it's "unofficial", but somehow refuses to use e.g. "EM64T" for it as well.