I've never seen this happen before, and I've watched the NBA since 1971. But for many of those years the team limit was 12, and all could play. There was no BS about selecting 12 of the 15 players.
1) In the early years of the 15-player limit, the 3 players were put on a fake injured list before each game, the Injured Reserve List. But I'm not sure the IRL began in exactly the same year when the 15-player limit began. When did this pre-game ritual begin...was it exactly the same year when the team limit went up to 15? (In the early years, could all 15 play?)
2) Has this ever happened to another team?
3) Why can't all 15 be available, and eliminate the pre-game requirement? Who cares if a coach rarely plays all 15 in a blowout; it won't hurt anyone.