1 They are in a hurry
If a person is in a hurry then they might end a phone call without saying buy. In such a case, the person might want to end the phone call so fast in order to attend to another matter.
2 They have Type A personality
Type A personalities are always in a hurry because of the personal deadlines they always have to meet. A type A might try to end a phone call very fast if they are busy or late.
3 They are close friends
People might hang out without saying goodbye if they are talking to very close friends. In such a case, knowing that the other person won't bother this action motivates them to do it.
4 They are impatient
Impatient people have low tolerance for formalities. An impatient person might hang up a phone call without saying bye just because they can't tolerate the long phone call. (See Why are some people impatient?)
5 They aren't that emotional
People who are not that emotional, those who aren't that sensitive and those who are not that considerate of other people's feelings might hang up the phone without saying bye.
6 They are rude
Some people might hang up without saying bye just because they are rude. In some cases, the person might also do this on purpose to send an indirect message to the one they are talking to. (See How to be assertive without being rude?)
7 They want the conversation to end
If a person got fed up of a phone call and they wanted it to end fast then they might hang up without saying bye when the conversation seems like it has ended.