Explanation/Reference:
An any-source multicast (ASM) network must be able to determine the locations of all sources for a particular multicast group whenever there are interested listeners, no matter where the sources might be located in the network. In ASM, the key function of source discovery is a required function of the network itself.
Multicast source discovery appears to be an easy process, but in sparse mode it is not. In dense mode, it is simple enough to flood traffic to every router in the network so that every router learns the source address of the content for that multicast group.
References: https://www.juniper.net/documentation/en_US/junos13.3/topics/topic-map/mcast-mvpns-draft- rosen-6.html