Slipping pulleys or gears or belts can cause the print to slightly shift and start leaning. Tighten pulleys/gears/belts to stop this happening. ( as you explain , you do it )
Friction along the axis might cause it to lose steps. Just a few steps here and there, enough to make it lean. Clean and lubricate the axis so there’s no friction.
A misaligned printer frame or bed will result in a leaning print as well, check everything is as straight as possible.
Check Current in your stepper drivers and tune it acording motor datasheet.
for the rest look on facebook , reedit, amino for other group about Anet because you are on ultimaker forums here , there is an example forum you can get more help https://www.3dprintingforum.org/anet 😉