50 MB is not much. Consider that for installation, the
apk gets probably temporarily stored there (in
/data/local(not always sometimes it's in the
/cache/download/ folder) -- at least that's what the playstore app does, I'm not sure about a "local install" where the
.apk probably is available from your sdcard) for processing, then gets installed (below
/data/app) and initializes its data (below
/data/data). And during all that, it should leave enough free space to allow all other apps to work normally: Creating temporary files, log files (at least the latter again being stored below
/data in many cases, e.g those you can access via
logcat), and manipulate their data (guess where -- yes, again below
So you see: Most of these actions take place on the
/data partition. Now look what size the
.apk of SBP Shell 3D has. Something about 10..15 MB? So when installed, it creates its data structure below
/data/data as well, which adds to this. And during all that installation process, free space probably falls below 30 MB (if it really requires a copy in
/data/local for the install, even below 20 MB) -- which is why the system alerts about "free space getting low".
The solution already came with the warning: Uninstall some apps. Or move some bigger ones / rarely used apps to SD, if possible. Or clean up some cache -- but that will only help temporarily.
You might also want to check some other space/installation related questions here:
could be good starting points.