All I have to say is, whoever teal was, he was pretty noob/moronic, it could just as easily have been reapers that jumped into his base and the same thing would have happened, except with Reapers it'd be more severe, whats surprising is the balls of the blue terran to do that. You'd really have to assume the person your fortressing has very little skill, and will likely ragequit to attempt it, there were just so many ways it could have been prevented, and even if it was, so many reason it shouldn't have made him leave, I'll list just a few.
1) Don't wall your base in completely as protoss, leave a gap and have a Zealot blocking it, and that will be enough.
2) If you see it coming, just plant a pylon down where it would land, those things have pretty short range when not upgraded, 6 I believe.
3) Rush his partner with your ally, Blue was practically finished and devoted it all to killing a nexus, while you can deal a heavy blow and possibly ending to his partner and eventually kill blue off for his weak tactic, its a team game, not 1v1.
4) If you know you can't stop it from completing but you see it land and begin transforming, save up your minerals or ask for some from your partner if you know you won't get 400 in time before your nexus is leveled and move base, the fortress can't lift off, and the blue terran doomed himself to sitting in a base he locked himself in with an out-of-position fortress to return minerals to.
In short, there was no reason to leave, if anything, it just garunteed teal's teams victory, because blue essentially took himself out of the game. Look at it this way.
If it was blocked it off from landing or finishing, blue would probably A. fly back to his base and go normal strategy and fall way way behind everyone else because of that, or B. leave it there and die trying to kill you off with SCVs.
If it landed and finished going fortress, you can pretty much assume blue is out of the game and sacrificed 1 Planetary Fortress (550 minerals, 150 gas) to take out 1 nexus (400 minerals) and can no longer move the fortress and has essentially become a giant turret in the base.
Either scenario, the game would have been over for Red and Blue.
