Maybe a distorted peak at the initial strike of the keys? Digital systems are 100% INtolerant to distortion. Older analog users know how to "warm up" a track by adding controlled distortion, but in the digital domain ..... there is NO SUCH THING. Distortion is always bad.

maybe the input signal is too strong, and you are compressing a spike? are you using the lite version of ProTools that comes bundled up for 6 or 7 hundred bucks? I have heard nothing but horror stories about those systems. A full blown, high end ProTools system is a joy to work with, but the baby version has too much fat trimmed off. Maybe the system is to blame.