Did you check the alert log.
Are you sure database did clean shutdown before backup started?.
might have waited for the job to complete...and this kind of case backup is no good.

to overcome this kind of issuses...(Ofcourse Big brains won't agree)
make shutdown script like below..

setenv ORACLE_SID $ORACLE_SID
$ORACLE_HOME/bin/svrmgrl < connect internal
shutdown abort
startup restrict
shutdown
#startup mount
EOF

you are fine.
Thanks.
Thanigaivasan.