Why are my adult dogs now barking when we put them in their crate at night?

What use to be a painless nightly ritual is now very painful! For the last two night they constantly bark when placed in their crate. I don't want to take them out for fear that it will encourage the barking whenever we use the crate. They are 4 and 7 year old Shihtzu. The 7 year old is the father of the 4 year old so they are used to each other. Please help me figure out a solution to stop this madness! I need a night of sleep...