Can anyone explain why I get these lines when taking photos in bad/weak lighting? Here are some photos.
These lines are very likely there because of the frequency at which your light flickers. This is most notably the case with fluorescent lighting or led lighting, especially when you dim the light. Led are dimmed by switching them on/off rapidly. The dimmer the light the longer the off periods which our cameras capture easily (and not only the X-T1)
Another possible explanation is that you used the electronic shutter; which can also cause interferences with certain light frequency.
You will find this issue well documented by the guys photographing performance on stage with led lighting; especially on performances with low lighting budget.
Hope this helps