What would cause drains to just a bath tub and a shower (in different rooms) to back up quickly then go down?

Check the vent pipe. The vent pipe allows air to enter the plumbing system and the water to drain smoothly.What would cause drains to just a bath tub and a shower (in different rooms) to back up quickly then go down?
could be the vent more likely hair build up in trap go down to lowes plumbing and get a tool called a zip-it its white and has barbed edges run this down the drain as far as you can give a twist and pull it out.if it is hair get ready for a shock if its hair if nothing comes out after a few tries run the water and get a plunger and plunge away.on the tub though you will need to cover the over flow with a second plunger or remove it and stuff wet rags in it so you can get a good suction.i unstop about 20 tubs a week as a plumber for our local university.....

wrinkles