Not a surprise really.
TC and tui are prone to this sort of thing (other operators are available).
You should remember that TC and tui are really package operators with their own airline to fulfil the package transportation costs under their own aegis, thus retaining any profit from those flights.
They then sell surplus space to third parties (like independent travellers and third party package operators) to fill the remaining seats, and maximise their bottom line (and ensure their 'airline' remains profitable).
IF, and it's a big IF, the package operator has low take-up of a particular flight for their packages, they will move the customers onto more convenient (i.e. profitable, by removing the flight costs from their balance sheet) flights, sometimes on different days, but more likely inconvenient (for the passenger) times.
This means the other passengers booked on the flight find their flight cancelled, or moved to a different time (often at night).
Obviously, if you fly with a scheduled airline these conditions do not exist, though there are some airline operators (you guess which) who, if left with a under-utilised flight will claim 'technical difficulties' and move the passengers to a later flight, or something. Their obligations are somewhat different to the charter flights, hence the discrimination between 'charter' and 'scheduled'.
In the end, if your flight is with a charter company, then these things are more likely than with a scheduled operator (though the caveat applies, this isn't 100% guaranteed - nothing is).
If you monitor far enough in advance your travel arrangements, you will notice that the aforementioned operators have dumped several flight in the last two months, in some cases leaving no flights at all from some airports, or reduced to one day a week. This, obviously can screw up your accommodation plans where you find your flight is now substantially different from your accommodation booking.
In the end, you pays your money and takes your choice.