Why does the Torah add the word "Pekudav" regarding the tribe of Shimon?
Rabeinu Bachye (20): It is an expression of 'Pekidas Avon' - Hashem visited upon the tribe the sin of Shimon their founding-father (who actually took hold of Yosef and cast him into the pit), and the Satan was able to reduce their number (in the census in Pinchas, after their Nasi sinned 1 ) to far less than half of their number here. 2
Zimri ben Salu sinned with Kozbi (25:14). Moshav Zekenim brings similarly from R. Yehudah ha'Chasid.