Didn't really think about it being possible for difficulty to decrease, but I guess theoretically it could.
It not only "could", it has.
I don't really understand why you say 20160 minutes though.
Difficulty is adjusted every 2016 blocks. Difficulty is adjusted to bring block creation closer in line with an average of 1 block every 10 minutes. If blocks are being solved faster than that, difficulty is increased proportionally to slow down block solving. If blocks are solved slower than that, difficulty is decreased proportionally to speed up block creation.
2016 blocks at 10 minutes per block = 20,160 minutes.
Is the minutes you are reffering to not a unit of time, but a different computing minute?
A unit of time, as in 1/60 of an hour, or 1/1440 of a day, or 60 seconds.