UKIP seems to have had a troubled relationship with a couple of its party leaders. Richard Braine, who led the party from August 2019, was suspended from the party while still in office over allegations of stealing data from the party. He officially resigned from UKIP a week later.
Additionally, while not a suspension/expulsion as such, Gerrard Batten, who led the party from April 2018 to June 2019, was blocked from standing for the leadership again in a motion entitled 'Gerard Batten has brought the party into disrepute by his actions and associations and should therefore not be allowed to stand for the leadership of the party' by the party's national executive committee.
Braine's successor, Freddy Vachha, who led UKIP from June 22nd 2020, was suspended from the party less than three months later, on September 12th, being replaced by former Tory MP and current UKIP Senedd Member Neil Hamilton. Vachha appears to be contesting the suspension. According to the party's statement:
Freddy Vachha has been suspended from membership of UKIP, pending a
formal investigation of a complaint of bullying, harassment, verbal
abuse and other conduct likely to bring the party into disrepute.
Suspension from party membership automatically suspends Mr Vachha from
the UKIP leadership.
It would be unfair to Mr Vachha give further details of the
allegations against him prior to the conclusion of the judicial
process. This will be conducted formally, impartially and subject to
the rules of natural justice.
On a more historical note, it turns out that this has actually happened before in the Labour Party itself. Ramsay MacDonald, Labour's first Prime Minister, was expelled from the party after forming the National Government of 1931 as a coalition of Labour, Conservative, and Liberal MPs.
Most of the Labour Party was horrified and, for putting the national
interest before party interest, MacDonald was denounced as a traitor
to his class and expelled.