Where does the CO2 in Mars atmosphere come from? As far as I know, Earth and Mars were relatively similar when the solar system formed. So most of the CO2 was used to form carbonic rocks (right?). I found that tectonic movements are the reason that the CO2 on Earth could be released again, which don't exist on Mars. 

So how comes that the atmosphere has 95% CO2?
The early atmospheres of Venus, Earth and Mars all had a lot of CO2, Carbon and oxygen are some of the most common elements in molecular clouds, and so it is no surprise that their most common and stable compound should be common in atmospheres.  

On the Earth, life happened.

The carbon in the atmosphere was fixed by autotrophs (particularly blue-green algae and the later plants) into the bodies of living things, and then into rocks formed from those bodies: Chalk, Limestone, Coal and Oil. This left an atmosphere dominated by Nitrogen, and later by the toxic waste products from those algae: oxygen.

Mars was too small to hold onto lighter gases like Nitrogen, and these escaped into space, there is just a trace of the heavier gases: CO2, but at a very low pressure.

Venus retains a dense atmosphere dominated by CO2