It is hard to provide good advice without adequate information. You must become a detective and determine what may have happened to influence this new behavior.
Horses seldom, if ever, change their normal behavior for no reason. This may be related to illness or injury. It may, also, be related to changes in environment such as something new which makes the area look different than it did. It may, also, have to do with a new and unusual smell or sound in the area of the cross ties. Finally, this change in behavior may be related to a recent negative experience that the horse relates to the cross ties. This experience may have happened while tied but it may be less concretely related to the cross ties themselves.
If you can determine the cause of the change in behavior, it is much easier to find a solution. If not, there are still things you could do. You would need to address the issue as if the horse had never been accustomed to the cross ties to begin with.
This second approach would involve experiment and observation. You would break the process down into steps and determine where the horse becomes reluctant. Then, you would try to determine the stimulus for the horse's behavior.