There are a couple of reasons why a player can have air escape through their nose while playing.
The first is medical. Sometimes an operation can change the air passages making it easier for air to escape. This happened to me. There are times where a new medical condition could cause this to happen. (Broken nose, nasal polyps, tumor ...)
Then there is the way in which you use the tongue to play.
There are and were teachers like Stevens who felt that arching the tongue as you played higher allowed the air to escape and was to be avoided.
I find that where you arch your tongue is most important. Arching with the middle of your tongue works to a point. However when it gets to the "EEEE" position then for many people it is so close to the sinus cavity that it can and does blow air into it. As you increase air speed and pressure to play higher then more air escapes.
The arch for higher notes should be made with the front part of the tongue. It should roll forward and help close the lip aperture. This act not only prevents blowing air through the nasal passages it also supports the lips, aids in their alignment, helps to cushion with air from behind and helps to speed up the airstream.
There are possible reasons why this may be showing up now. It may just
be that you have recently worked on better breath support or worked more
on tongue arch. Even things like weight gain or loss can affect this.
Clint 'Pops' McLaughlin