Because you know that the discombomb can make people "fly" away. And because you throw it into an area where you have no idea/control with whose there it's up to you to then make that choice to take the risk or to leave it.

In any case, you threw it and that's up to YOU. Not the other person so it was RDM due to the fact that you had no idea who else was there and you threw it anyway with the chance that someone else could get hurt.

Because you know that the discombomb can make people "fly" away. And because you throw it into an area where you have no idea/control with whose there it's up to you to then make that choice to take the risk or to leave it. In any case, you threw it and that's up to YOU. Not the other person so it was RDM due to the fact that you had no idea who else was there and you threw it anyway with the chance that someone else could get hurt.