The firsat time I flew as a passenger on a EMB145, i noticed the same thing. I've been flying them from the front for about 3 months now and don't notice it at all.
What I was told in groundschool/conversion training was that it was caused by the aircraft being very slightly out of balance.
The aircraft yaws slightly and the only wa the autopilot can correct is by using the ailerons.
We are flying at high altitudes and M0.76, but only have outboard ailerons, no high speed inboard ailerons. The ailerons are either not stiff enough or they cause the whole wing to twist slightly and then you get the roll you experienced.
Solution - keep the ball in the middle!