What causes a lipo battery to become unbalanced?

One of the lipo batteries on a 4S mod of mine has one cell that has a lower voltage than the others and will only charge the battery to 70% of the capacity.
Here's a pic of EScribe, cell number 2 is lower.

View attachment 948739

What is the cause of that cell being unbalanced?
I'd rather avoid any further hassleso_O
@Mooch any lipo info would be very much appreciated Mate:thumb:

Original source: https://www.e-cigarette-forum.com/threads/what-causes-a-lipo-battery-to-become-unbalanced.965949/