Yeah, you can see that by looking for when companies do open-source old code. It's usually a case where someone has some specific reason they want it open source, and is willing to push for it.
id Software games: John Carmack wants to get old code out there, partly so students/others have some real-world examples to peruse and hack on rather than only toy games from textbooks, and partly because he's proud of a lot of his clever hacks and eventually wants them to see the light of day.
Axiom (computer algebra system): Someone starts a proprietary software company based on academic research, and it fails. One of the original researchers has a personal interest in getting the code released, so the project can continue.
Blender: Company goes bankrupt, and the original programmers want it to be open-sourced so their work doesn't disappear. Raise $100k to buy it from the creditors in bankruptcy.
id Software games: John Carmack wants to get old code out there, partly so students/others have some real-world examples to peruse and hack on rather than only toy games from textbooks, and partly because he's proud of a lot of his clever hacks and eventually wants them to see the light of day.
Axiom (computer algebra system): Someone starts a proprietary software company based on academic research, and it fails. One of the original researchers has a personal interest in getting the code released, so the project can continue.
Blender: Company goes bankrupt, and the original programmers want it to be open-sourced so their work doesn't disappear. Raise $100k to buy it from the creditors in bankruptcy.