When I was playing Team Fortress 2 I got to charge the grenade launcher of the Demoman class. When I hold the left button, Demoman puts a lot of ammo on the launcher and release them all at once. But then when I played again I can't do it anymore.
The demoman has no ability to fire multiple bombs at once but can detonate many or all grenades at once using secondary fire.
He can charge shots by holding down primary fire. On primary fire release he will fire the bomb a distance proportional to the amount of time primary fire is held down. This only works with the three sticky bomb launchers.
No Demoman weapon in Team Fortress 2 can fire multiple projectiles simultaneously, so it's unclear what it is you're looking for.
The Beggar's Bazooka for the Soldier functions as you describe. Perhaps you were actually playing Soldier?
(I've got a dim memory of seeing a video where someone did do that, but I don't recall any more details. Still, it must have been a mod or something.)
Sorry, I know this has already been answered, but I came upon the same thing while playing on the 10x servers. The 10x servers add a great deal of functionality to to weapons, including the specific effect noted in the question.
A complete list of changed on these servers can be found on the tf2 wiki: http://wiki.teamfortress.com/wiki/TF2_x10