Very difficult to say. If possible, take a photograph next time it happens.
From my experience, Oscars will go through a kind of ritual before laying eggs . If you've got a rock in your tank for instance, they may choose this to lay their eggs on, they will probably start cleaning it it vigorously a couple of days before laying eggs. At some stage, you will see their breeding organs appear. This could be the day before, or on the day that they lay eggs.
I can't have ever noticed my Oscars breeding tube sticking out when they have never laid eggs. One of my females breeding tube is visible all the time, but it's not sticking out as far as it would do if she lays eggs.
There out some other members who have got breeding Oscars so no doubt they will give you their opinion as well.