Paul is correct. Carrara assigns a default UV map upon object creation based upon the geometry. Since booleans fundamentally alters the geometry, the UV map becomes useless be cause it is trying to map the old coordinates to the new geometry - which doesn't work.

This is true for all 3D software. 

To fix this issue, open the object in the vertex room and engage the UV mapping options. There you can realign the geometry to the UV space.