Noticed my very last left molar is shifting to the left but others are in line with each other? how can this happen? biting left cheek making mark

Wisdom tooth: Wisdom teeth are last teeth to erupt and some times there is no space for them so they deviate from the normal position. In that case they need to be pull out otherwise they will have problems. Please consult with your dentist to guide you.

Possible cross bite: If the tooth is not in good occlusion or alignment with the other teeth, this shifting could occur. Some one who bruxes or clenches their teeth can cause this, and you need a guard. See a dentist for a professional guard, or try an otc guard.

Could be your wisdom: Many times these teeth as they erupt further may shift to the cheeks side or tongue side as there is inadequate space to fully erupt into a normal occlusal ( bite) relationship.

Slow movement: Although unlikely, it is possible that a molar may move - slowly - into a position that you've noted. There may have been an "end-on" position of the points on the top surface of the teeth the slowly shift perhaps into a "cross bite." best to see an orthodontic specialist to see what can be done to correct it.

Crossbite: It sounds like you are describing a crossbite. That means your upper back teeth are on the wrong side of the lower back teeth. Sometimes this can easily be fixed using rubber bands.

Lots of reasons: Probably position of upper and lower opposing teeth was not optimum, and over time the problem becomes more pronounced. Check w your Dentist. If this is the case you may need to see an Orthodontist. Back molars can also move out of position because of malposed impacted wisdom tooth, presence of a boney lesion, or gum problems. If so your Dentist may want to refer you to an Oral Surgeon.