Why is 18 month old being tested for angelman syndrome?

Because. Most likely due to physical features and some developmental issues. These kids often have some delays. It is a genetic condition due to a defect in the child's chromosomes.
Severe speech and DD. Affected children have unique presentations, usually by age 2. They do not walk until 2 to 6 yr. Speech is severely delayed, and may never develop. Gait or movement of arms may be unsteady or tremulous. The unique finding usually occurs after 1 year, with an inappropriate happy demeanor, laughing, smiling and easily excitable. Many other signs, but the above is sufficient to initiate testing.