Why query plan changes by number of rows

I execute following two queries on an oracle database:

select * from qsn join plc on plc.fk_qsn = qsn.id where rownum < 48507

which returns result in less than a second and with following execution plan

and

select * from qsn join plc on plc.fk_qsn = qsn.id where rownum < 48508

which takes more than 10 seconds to execute and uses following execution plan

Can anyone help me understand the reason why this strange behavior happens?