Indeed, many airlines state that if there is no more space left in the overhead lockers, any remaining cabin bags must be checked.
For example, according to easyJet’s website: “Overhead locker space is allocated on a first come first served basis and once the overhead lockers are full all remaining customers will be asked to put their cabin bags in the hold free of charge as long as their cabin bag does not exceed the maximum cabin bag size of 56 x 45 x 25cm.”
Although at this point, a mandatory hold check-in tends to come at no extra cost, many travellers prefer not to be parted from their belongings.
However, the mandatory check-in can be a draw for some.
One commenter said they always board last so that their bag can be checked for free.