The code is using btMatrix3::setEulterZYX(x, y, z) to build a 3x3 matric whose values represent a rotation of PI/2 (aka 90 degrees) about the z-axis. The rotation is being used to initialize the transform of the btRigidBody that corredponds to the upper left arm.
The upper left arm is probably using a btCapsule shape, which means by default its axis lies parallel to the y-axis (up). After a 90 degree rotation about the z-axis (using the
right hand rule) the capsule should lie parallel to the x-axis, pointing in the negative direction.
Try changing the angle value from
1.0 * M_PI_2 to be
0.5 * M_PI_2 or
0.25 * M_PI_2 to see how it changes the position of the left arm.