When someone creates a thread on a topic, especially one where the person is seeking help or advice, changing the topic to something about yourself or your own questions and deviating the thread from it's original intent is hijacking. How can the person who created the thread get what they came looking for if you take it over?
This is less serious on more laid back thread topics. But ones where it's more serious or someone creates it for their own needs, it's just rude and inconsiderate and frowned upon by most people. Is it really hard to understand that taking over something someone else created, and making it about you, is not a right thing to do and you need to see a posted rule for it to make sense?