Definition - What does Software Propagation mean?
Techopedia explains Software Propagation
If a programmer takes GNU code and alters it, he is allowed to share it (propagate) but he has forfeited legal rights to it by doing so. If the programmer makes a claim of ownership, the original license he agreed to in order to get the code strips him of his rights to the code within 60 days of a violation (in this case, the ownership claim). The programmer is left owning the tweaks to a code base that he has no right to, thus preventing any commercial advantage to claiming ownership.
The GNU GPL already provides users the right to modify and execute the code as they see fit, so the software propagation language was added to head off any attempt to claim ownership when/if those modifications were shared.