Perhaps another way this could happen might be by somewhat widespread use of something along the lines of
================= Cisco Version ========================
route-map CIDR_Top10 deny 10
match as-path 10
match ip address prefix-list deagg_pollution
route-map CIDR_Abusers permit 1000
ip as-path access-list 10 permit _4323$
ip as-path access-list 10 permit _18566$
ip as-path access-list 10 permit _4134$
ip as-path access-list 10 permit _721$
ip as-path access-list 10 permit _22773$
ip as-path access-list 10 permit _7018$
ip as-path access-list 10 permit _27364$
ip as-path access-list 10 permit _6197$
ip as-path access-list 10 permit _3602$
ip as-path access-list 10 permit _6478$
ip prefix-list deagg_pollution seq 10 permit 0.0.0.0/1 ge 21
ip prefix-list deagg_pollution seq 20 permit 128.0.0.0/2 ge 21
ip prefix-list deagg_pollution seq 30 permit 192.0.0.0/3 ge 24