It is a feature built into the firmware. The surging is extra effort to get the wheel under you and prevent a nosedive if it senses you are starting to push it toward the limit. Pushback is a warning when you are gradually pushing the limit of speed and available power. Surging is a warning that you might be aggressively pushing that limit. You are more likely to get pushback on level ground, and surging when going uphill, over bumps, or anything else that might suddenly draw a lot of power.
If you are feeling that momentary surging, slipping feeling, back off.