I heard that it is because they couldn't decide how to divide them between the heirs to the estate. Apparently because J-C wasn't too keen on M-T, the daughters didn't want any personal items to fall on her hands and opted to sell instead. So incredibly sad to think about that; going under the hammer are such items as GDss J-C's wedding tiara, a necklace that was given to Queen Astrid (her mother) as a wedding gift by hubby Leopold III and many other items of historical value too.
Ah well, royals are people like everyone else
"Contrariwise", said Tweedledee, "if it was so, it might be; and if it were so, it would be, but as it isn't, it 'aint. That's logic."
From "Through the Looking Glass" by Lewis Carroll