Why my phone is always out of space recently, but I didn't install any new software
It should be the applications occupying /data/ space as they are in use, mainly due to runtime files, cache, database, etc.

In order to verify this, you can execute the following:

adb shell
su -c "du /data" | sort -n


It should have a result like:

...

3792    /data/data/com.UCMobile
28399   /data/app
35647   /data/data
49374   /data/dalvik-cache
114865  /data/


And then you can check the directories from large to small, to see what's taking so much space.