I believe the most common reasons for attorney disbarment are: 1) Stealing from their clients, usually by stealing from their client's trust account, 2) Being convicted of a major crime, 3) Multiple--emphasize multiple--instances of client neglect or failing to represent a client in a matter the attorney promised to do so.
Because a law license is so valuable in terms of earning a living, the bar organizations of the different states must have a strong case for disbarment. Otherwise the state supreme court which usually has the final say will side with the attorney resisting disbarment on fairness and due process grounds.
Short of actually stealing from the client (this doesn't include charging excessive fees) or conviction of a major crime, most other complaints of attorney misconduct result in lesser sanctions, including, in order of increasing severity: private reprimand, public reprimand, and suspension. Of course, if an attorney keeps getting disciplined for a series of misconducts, none of which by themselves would get the attorney disbarred, the accumulated weight of all of these matters may get the attorney disbarred.