Oracle Database Discussions

Hi,
Oracle instance got started automatically and in the alter log file i found the information as below with no error or warnings.
Thu Mar 25 02:45:31 2010
Thread 1 advanced to log sequence 14179
Current log# 2 seq# 14179 mem# 0: /u01/app/oracle/oradata/DBNAME/redo02.log
Current log# 2 seq# 14179 mem# 1: /u01/app/oracle/oradata/DBNAME/redo02a.log
Current log# 2 seq# 14179 mem# 2: /u01/app/oracle/oradata/DBNAME/redo02b.log
Thu Mar 25 02:51:32 2010
Thread 1 advanced to log sequence 14180
Current log# 1 seq# 14180 mem# 0: /u01/app/oracle/oradata/DBNAME/redo01.log
Current log# 1 seq# 14180 mem# 1: /u01/app/oracle/oradata/DBNAME/redo01a.log
Current log# 1 seq# 14180 mem# 2: /u01/app/oracle/oradata/DBNAME/redo01c.log

(NOTE:From Thu Mar 25 02:51:32 2010 to Thu Mar 25 07:31:24 2010 nothing has happened on the DB.)

Thu Mar 25 07:31:24 2010
Starting ORACLE instance (normal)
LICENSE_MAX_SESSION = 0
LICENSE_SESSIONS_WARNING = 0
Picked latch-free SCN scheme 1
Using LOG_ARCHIVE_DEST_10 parameter default value as USE_DB_RECOVERY_FILE_DEST
Autotune of undo retention is turned on.
IMODE=BR
ILAT =61
LICENSE_MAX_USERS = 0
SYS auditing is disabled
ksdpec: called for event 13740 prior to event group initialization
Starting up ORACLE RDBMS Version: 10.2.0.1.0.
.....
.....

I do not know what makes oracle instance to start automatically, can some one please let me know what is the exact problem behind this conspiracy.
DB Version:10g R2 64-bit
OS:LINUX
Release: 2.6.18-164.el5
Machine: x86_64

Thanks.