Why do some guys get mad when you don't text back?

I've noticed that a couple of my previous boyfriends and the boyfriend I have now kind of get upset when I don't text them back after a few minutes of sending me a text. One of my ex's even threw a fit one time when I didn't have my phone with me and I didn't answer for about an hour and he texted me like 10 times. After I called him to explain, he gave me the third degree. The boyfriend I have now just gets upset when I don't text him back immediately. He'll text things like "Well I guess you don't want to talk to me right now" or "fine. don't talk to me" One time he texted me wondering why I hadn't talked to him.

This only happens with texts. I mean when we talk on the phone or in person, none of this is ever mentioned. So why do guys act like babies and freak out if you don't text them back immediately? I don't try to ignore them. I just don't always have my phone in my hand at all times.