What would be possible excuses to imprison a royal consort?
If the queen consort is cheating on the husband, this is treason (because then her children might not really be "his"). That's one.
Eleanor of Aquitaine ended up in confinement for rebelling, but she had her own powerbase, so I'm looking for someone else.
Any others? How about espionage? The Queen consort of England giving court secrets to France... actually most of the court proceedings were open or open secrets so maybe not?