Thomas Cook offload 100 passengers
 

Article here

See inside 'ghost plane' after more than 100 passengers got off before it was declared safe to fly - Gazette Live

Any idea why they had to offload 100 passengers to be safe to fly?

The article states it was due to an earlier technical fault, so one would assume there had been some kind of fault regarding a technical aspect of the plane.

That aside, what kind of idiot puts all his keys in his checked in baggage? What goes through peoples heads?

Bit daft, but I could see it's the sort of thing I could do. Chuk em in the suitcase when you get to a destination as I won't need them and won't then forget them. Then have to hope I remember to get them out before travelling..

The only things that spring to mind are two possible scenarios:

a) One or more doors or escape slides inoperative
b) One or more cabin crew short of required crew complement (sickness; injury etc?)

Airline would not choose this option lightly so there may be other reasons but right now can't suggest. So much easier if airline explains honestly at the time.

...or performance or loading limitations. But an open and honest description of the problem, the effect and the solution and timing if possible.

Starbear
 

There are a few more that you missed.
1. Error in the load sheet so a/c is way overweight due to the amount of fuel uploaded.
2. Incorrect fuel amount uploaded.
With both issues you would not be able to remove the fuel, so the only way to reduce the fuel figure would be to burn it off.
The only way to do this would be high power engine runs, only if you could find a suitable area at the airport to perform them. However if this flight was a night flight most airports would not allow that solution. Even if you could get permission to do so it might take time and the crew might not have enough duty time. In this case it would be better to remove the 100 pax and operate back to the UK, thus saving the company a shed load of more EU 261s.