Probably because you still had your accounts set up in OE and you opened it.
Either delete the OE accounts, configure OE not to send/receive - I assume
thats possible, I've never used it -
Ensure your send/receive options are correctly configured in OL, do NOT set
to check for mail at less than 10 min intervals.
Ensure OL is set as your default mail app.
For any other Q, please reveal your version of OL