As DWF said, Terror of the Zygons was already in pre-production as the season 12 closer before the decision to end season 12 early so season 13 could start in September 75 rather than January 76 was made (but they knew by the time it was actually made).
A surprisingly large number of seasons for Doctor Who's original 26 year run shot material for the next season at the end of the previous one, by accident or design:
Season one: Planet of Giants and Dalek Invasion of Earth were scripted as stories nine and 10 of Doctor Who's one-off 52 week run, but were then held back so season two could start earlier (they ran from October to Christmas at the start of season two, rather than July to October at the end of season one, then season two-as-produced started in January with The Rescue as it would have done anyway...).
By the time they were recorded this was already known, so the last episode of The Reign of Terror ends with a season-capping discussion between the Doctor and co, but Planet of Giants was still recorded as originally planned, and then re-edited to tighten it up so it'd be a [slightly] better season opener.
Season Two: Galaxy Four and Mission to the Unknown were shot at the end of season two, and Mission was planned as an end of season teaser/cliffhanger, but again the decision to change things had been made before the actual recording, so a 'season ender' montage was added to the end titles of the Time Meddler.
Season Three: The Smugglers was shot at the end of season three, but was always intended to be held back to open season four.
Season Four: Again, Tomb was made at the end of S4, but always intended as the season 5 opener.
Season Five: The Dominators and The Mind Robber were both made at the end of season five, so that season six could start unusually early before going off air for a few weeks while the BBC broadcast the 1968 Olympics between Mind robber and Invasion.
Season Nine: Carnival of Monsters was made for season 10 at the end of S9. Season 10 production began with Frontier in Space, before heading back to make The Three Doctors (during the only empty space in Patrick Troughton's schedule). But having established this pattern...
Season 10: Ended with production of The Time Warrior for s11.
Season 11: Ended with production of Robot for S12: Actually, Tom Baker was doing location work for Robot while Jon Pertwee was still in studio on Planet of the Spiders... reportedly Pertwee was not happy about this - too much of a reminder that he really was replaceable.
Season 12: As mentioned, re: Terror of the Zygons, but they then went straight on to make Pyramids of Mars for the next season anyway. And in practice there wasn't any real gap before production on the rest of season 13 started with Planet of Evil... Which was fine on the newcomers, but Lis Sladen barely got a day off in two years!
After this, it settles down a bit. until:
Season 18: ended with production of Four to Doomsday and The Visitation, as Peter Davison had already been booked to record a final season of Sink or Swim before he was cast as the Doctor, so they had to cram two stories into the gap he had before that.
Season 19: With two stories already in the can for season 19, it made sense to make a full 26 episode season and get Snakedance and Arc of Infinity made ahead of time for the next run.
And then it's all as transmitted (but not always in order within season).