1) First install NppAutoIndent plugin if you don’t have it.
Plugins > Plugin Manager > Show Plugin Manager, then install NppAutoIndent from the “Available” tab of that menu.
2) This behavior can be turned off by doing:
Plugins > NppAutoIndent > Previous line
3) If this option is disabled, you may need to first check this option:
Plugins > NppAutoIndent > Ignore language