A topic automatically becomes "hot" after a certain number of posts. I think when it reaches 16 posts it becomes "hot". Why some topics are red hot and some are a duller red hot is because the hot topics you haven't viewed yet are bright red and the one's you have viewed are dull red.
To get a topic that will attract lots of people, you must make a topic that will attract a lot of people because they will get something to gain by posting or viewing the topic. Whether they gain entertainment, a satisfaction of curiosity, or something such as material that may benefit them now or in the future, you must get their attention. Sometimes, topics become hot because lots of people click on them just because they don't understand what the topic is about. I am NOT suggesting making confusing topic titles so people will click on your topic to figure out what the heck it's about, but that is one reason why topics become so hot. Only experience will allow you to create hot topics. If you post something that people can't really respond to or something that most people don't have common grounds on, you can't expect it to ever become hot.