Player A rolls a 7. Player A has to discard cards, and does so quickly. Player B also has to discard cards, and doesn't, allowing clock to run out. On timeout the game discards for Player B, and at the same time automatically places the robber on Player A's behalf, preventing Player A from choosing their own placement.
If card discard timeout is due to another player, then the player that rolled the 7 should be given time to place the robber.