What are the main causes of loose connections in lighting circuits?
Your question is not unsuitable but it is a little puzzling. The main reason for loose connections is improper installation. Connections, splices and taps should be mechanically and electrically sound (that's right out of the training manuals). The fact that something can be used before failure, does not mean it was originally installed properly. It just took time for the fault to become evident.  

I think what you should be looking for is evidence of improper or loose connections. That would be discoloring, burnt insulation or smell, and loose terminals. Of course electrical evidence would be flickering, equipment seeming to struggle to come on, an overheating smell or hot to the touch. An infrared thermometer is very useful in in detection hotter than normal surface areas. 

Hope this helps to educate and good luck.