Galvanic reaction between aluminum and carbon fiber composite
The reason is that aluminium carbide, $\ce{Al4C3}$ is formed. The reaction occurs at around 500&deg;C on the interface. The problem with this substance is that it is brittle, and it also reacts with water:

$\ce{Al4C3 + 12H2O -&gt; 4Al(OH)3 + 3CH4}$.

More info at this and this wikipedia page.