SQL & PL/SQL

In oracle 10.0.1 version, There are a lot of oracle.exe(SHAD) process using SQL:
select * from v$process;

Because the number of process exceeds the limit, it prompt error "ORA-12516". A few days ago, I have increased the limit value of process and it works well. But now, I don't know who modify the system or configuration of Oracle so that "ORA-12516" occurs. Moreover, I doubt the reason is not the lack of process limit and maybe some programs have not released the session or process?

Thanks a lot.