I've been flying my 400 for about 2 months now almost every day and have been very happy with the way it has performed. I just started experiencing a little bit of tail wag the last 2 flights. I've never had any wag whatsoever before this. What could be possible reason for this to start happening after so many flights?

Just turn the gain down useing your dx6, just a little.

Check for any binding anywhere in your tail mechanical setup. Also check for a bent tail rotor shaft.

If you haven't changed your flying style and the tail just started wagging, I'd suspect some sort of binding in the servo linkages or the tail rotor pitch slider. Should all be very, very smooth. The servo / gyro will sense any binding and attempt to counter the jerky movement. That looks a lot like wagging but truly isn't.

Wagging is caused by gyro gain being too high and the gyro getting out of phase with its own inputs. So, lowering the gain will reduce the wag if it is truly wag but will also limit the ability of the gyro to keep the tail / heading steady. You want the gain as high as possible without wagging in your intended flight modes.

You might also try upgrading the gyro to a DSP75. This will allow you to use the high resolution output from the gyro and the servo will respond much more quickly. You might find that you need less gain with the more responsive gyro but heading hold should be much improved. There is lots of good info on this forum about the DSP75.

Hope that helps!

Check your wire connections.