Why did Johnson, Nixon, and Clinton faced impeachment?

Andrew Johnson, not Lyndon, was impeached for continuing Lincoln's reconstruction policies in the South. However, members of Congress were so enraged of the South's secession and eventual war that they wanted to punish the South and regarded Johnson as weak. The Senate found him not guilty by one vote. Nixon was never impeached for the Watergate cover-up and pay-offs, as he resigned before the impeachment trial in the House of Representatives even started. Clinton was impeached for obstruction of justice regarding his incident with Monica Lewinsky in the Oval Office. He lied to federal investigators about the incident. While the House impeached him, the Senate did not find him guilty.