People who inherited a house but don't live in it (wrong size, wrong location, already have a house). They may wish to keep it rather than sell it, or wait and see how they feel later - there could be inheritance, tax or other issues.
They can sell the house, they just don’t want to: not accidental landlording is it?
People who have relocated within the country or abroad, but are intending to come back.
They can also sell the house: landlording is a choice, not an accident.
People who can't sell their house.
Bet they can sell their house, they just don’t like the price at which they can sell it.
People who have accommodation with their job and intend to retire to the house.
They can sell the house and invest the proceeds in a pension, what with pensions being intended for retirement and houses not being intended as a retirement plan. Again, it’s a choice.
Look, landlords are gonna landlord, but the whole “oh this is accidental” and “oh I’m so reluctant” is bullshit. Own the decision.