This all depends of the person. It's liek how some people get sea sick and how some don't. Sometimes it can be because of reading, eating, or using a mobile device. You can search google for further information on this. I'm not an expert on this, but I know there are some fellow friends that can help you on google!
Everyone is different! Say they ate alot, wasnt felling well before, like me been in the same spot too long,and If they are reading constantly..Well i suggest putting the book up.Believe me.. you don't want to exsperience it! If they let you know they are going to get sick,then pull over!