this is always true for elastic oblique collisions...
the velocities of the balls along the common normal dirn get exchanged and hence the 1st ball comes to rest in this dirn (retaining its velocity component along the tangential dirn) while the other ball acquires the normal velocity..... hence they move perpendicularly......
Mathematically it can be proved by writing any two of momemntum conservation (along normal), KE conservation and coeff. of restitution...