There will always be some arcing on a commutator - the thing the brushes press against - sometimes it's possible to clean it up to make the interference between the pieces run more smoothly - when I do it I use very fine sandpaper and make the commutator shine - but I still get a few tiny sparks.
https://www.youtube.com/watch?v=MkBLwUG_AdU
https://www.youtube.com/watch?v=gc4l1eooPKM
What could be the cause?
It might be a dirty or damaged commutator.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix lawn mowers.