Toilet backup for no apparent reason ?

I have 2 toilets in my house (single story on slab - if it matters). On of them seems to have a tempermant! On occasion, the toilet will back up... I've seen this happen when there was no waste to back it up. I'm wondering if the air vent is somehow obstructed??? What other circumstances could cause this?

Thanks!

toilet

The sewer branch to the toilet is plugged.

Sounds like I should get a plumber for this one... Thanks for the help