It is not unusual for a trust document to have several amendments to the trust provisions during the life of the trust. Once a trust becomes irrevocable, amendments still can be made, but usually only with a court order. the review of the trust usually is made every 5 years, or when a life event such as death, divorce, disability or marriage happens. And unfortunately it seems as if a person goes into the assisted living or hospital, the family finds the trust and attempts to make last minute changes. Last minute changes to the trust create unique judgement calls as to competency and influence over the changes being made. Independence in the trustee selection is often needed to assist with a emotional situation.
Changes in tax laws can be another reason to make changes. In recent years the estate life time exemption amount has changed several times, and often the provisions of trusts need to be made to reflect the change in the tax law.
Changes in assets, the selling of property, acquiring property, gifting property, etc., can change allocations among beneficiaries, that will require an amendment.