Like the first regeneration Hartnell to Troughton, the Troughton to Pertwee regeneration was forced on the regeneree. The Time Lords had investigated the doctor and found him guilty of stealing a TARDIS and of sticking his big nose into situations where it didn't belong and among his punishments was a forced regeneration. A curious punishment. Does it constitute an execution? Or is it more like a jail sentance that takes years off the person's life? That would be the case if each regeneration was limited to a certain number of years, but was anything ever said about that? It sounds like a board game where the Time Lords have taken away one of the Doctor's "Die Free" cards.
Another part of the punishmment meted out by the Time Lords was that the regenerated doctor was confined to Earth and in a related punishment, his knowledge of the TARDIS was taken away. So he still had his TARDIS, but he no longer know how to use it. Bummer.