Why does NO2 react with HCl ? What is the mechanism of the reaction?
Yes, I believe it is a typical redox where nitrogen dioxide is an oxidising agent and hydrochloric is a mild reducing agent as Ivan noted.

The reaction proceed in two phases:


$$\ce{NO2 + 2HCl -&gt;[\Delta] NO + Cl2 + H2O}$$



  Nitrogen dioxide react with hydrogen chloride to produce nitric oxide,
  chlorine and water. This reaction is proceeds by heating.(source)


Now some nitrogen monoxide react with water to form nitrosyl chloride.

$$\ce{Cl2 + 2NO → 2NOCl}$$


Finally, we get our main reaction


$$\ce{2NO2 + 4HCl -&gt; 2NO + 2Cl2 + 2H2O ~~~~~~ (1)*2}$$

$$\ce{2NO2 + 4HCl -&gt; (2NO + Cl2) + Cl2 + 2H2O}$$

$$\ce{2NO2 + 4HCl → 2NOCl + 2H2O + Cl2}$$


  Nitrogen dioxide react with hydrogen chloride to produce nitrosyl
  chloride, water and chlorine.(source)