It seems to be when an individual promotes what they are reading. If you were going round other JW's promoting some other Christian interpretation and were quite adamant, you will quickly draw the attention of the elders.
Knowing their policies inside out. For example, I told my Mrs that I could guarantee her that I would be spoken to if I grew a beard. This I did. Within 3 weeks the cobe pulled me to the back of the hall and had a chat with me as to why it was important to follow the example of the brothers on the platform at assemblies when it came to grooming.
Another time an elder approached me asking if my niece (who was staying with us at the time) was getting serious with an unbaptised brother as the boe were a little concerned. There antennas are always on when it comes to their policies and rules.