This is a control mechanism used for strafing and is deliberate, although it is annoying since there seems to be little ryhme or reason to exactly what the enemy does, so they tend to charge at your heavily armed assualt team with mellee weapons making the controls go crazy.

I believe there is an option to change this however labeled something like "always face forward/target" or some such, just play with the options until you find it.

To explain the mechanics behind it:

after targeting something/someone the camera focuses on them and the controls will always be relative to there position, so left and right strafe instead of turn etc, to help keep the enemy under focus.