PBKDF2+HMAC hash collisions explained

Some PBKDF2-HMAC-SHA1 collisions that start with the URL of this Hacker News thread:

    $ ./brute-force.py 'https://news.ycombinator.com/item?id=7465849#'
    https://news.ycombinator.com/item?id=7465849#aaaaaaaaaaaaabufhkcn 💥 VID9Qf*}8w9sBtI@TK5k
    https://news.ycombinator.com/item?id=7465849#aaaaaaaaaaaaabwwhwta 💥 Bgvi4F~6M#utux ~\H4m
    https://news.ycombinator.com/item?id=7465849#aaaaaaaaaaaaacjypwdz 💥 Tn/ZyN'Zs6?d){Ytd?}i
    https://news.ycombinator.com/item?id=7465849#aaaaaaaaaaaaaewyefbo 💥 r;9U\DZh!2de#bt <2*o
    https://news.ycombinator.com/item?id=7465849#aaaaaaaaaaaaaezrsysb 💥 wl)@eb.1Avvp?|NtT(\c
    …