Leaving a grass strip can occur after maintenance or during your mowing session. If your lawn mower blades contact something such as a stump, root or rock, it can bend the lawn mower blade, the blade adapter, a deck spindle or crankshaft in the case of a push or self-propelled lawn mower.
You could also have a loose lawn mower blade and this will create vibration as well as an indication. Check all of the above items to see if they are the cause of the problem and replace any parts to correct it.
Another item that can cause this issue is that your deck is out of adjustment. If the deck was recently removed, make sure that both rear mounts are in the same holes on both sides. Also check the blade height by measuring from the ground to the tip of the blade on the outside and then measure the opposite blade. If one blade tip is higher than the other, then the deck will need to be adjusted to make sure they are both level.
The deck should also have a slight downward angle towards the front of the lawn mower. Think of how a helicopter is in flight with its blades having a downward angle to bite the air.
After you make your adjustments, test mow a section of your lawn to ensure that your changes have resolved the problem.