The TARDIS knows how Donna was fired, it knows she's a bit clutzy, and it knows she takes subliminal suggestion, especially after being recently demetacrisised. It also misses her and needs her to stay with the Doctor for a while longer. So, predictably, Donna spills her coffee on the console, and the TARDIS is all like WHOOAAAA, oh my, I'm falling, I'm on fire, look at the sparks, we could go anywhere!! Heehee.
Also, I think 14 is the first regeneration since the show came back who didn't immediately blow the TARDIS up upon regenerating, so it had to take care of it by itself!
Here's how I interpreted that scene:
The TARDIS knows how Donna was fired, it knows she's a bit clutzy, and it knows she takes subliminal suggestion, especially after being recently demetacrisised. It also misses her and needs her to stay with the Doctor for a while longer. So, predictably, Donna spills her coffee on the console, and the TARDIS is all like WHOOAAAA, oh my, I'm falling, I'm on fire, look at the sparks, we could go anywhere!! Heehee.
Also, I think 14 is the first regeneration since the show came back who didn't immediately blow the TARDIS up upon regenerating, so it had to take care of it by itself!