a few things can be the culprit.
one is your scroll lock key on the keyboard(the one your typing on) see the scroll lock key ?
sometimes we hit it by mistake.
2) theres a setting in the track view controls under views.
this will stop scrolling or start scrolling options.
3) its most likely this one...if you make an edit while sonars nowtime is moving you will freeze the scrolling and it will lock up.to solve this just right click on the timeline where your magnifying glass appears when your using the zoom feature.
sonar does this on purpose because it thinks you want it to stop scrolling so you can edit that part of the song.
there is also a setting in the trackview to make scroll locking during edits or keep moving.
there under views.
good luck.