Unable to drop non-PK index because it is referenced in a foreign key constraint
Because a foreign key can point to a primary key or a unique constraint, and whoever created that foreign key possibly created it before the primary key existed (or they shifted the FK to point to the Unique index while they changed something else about the primary key). This is easy to repro:

CREATE TABLE dbo.MyTable(MyTableID INT NOT NULL, CONSTRAINT myx UNIQUE(MyTableID));

CREATE TABLE dbo.OtherTable1(ID INT FOREIGN KEY REFERENCES dbo.MyTable(MyTableID));

ALTER TABLE dbo.MyTable ADD CONSTRAINT PKmyx PRIMARY KEY(MyTableID);

CREATE TABLE dbo.OtherTable2(ID INT FOREIGN KEY REFERENCES dbo.MyTable(MyTableID));


In fact, both of these foreign keys will point to the first unique constraint defined on that column (myx).

You can fix the foreign key on the other table by dropping it and re-creating it. You will need to repeat that process for any other tables that point to this column. You can find these easily:

SELECT s.name,t.name,fk.name
FROM sys.foreign_key_columns AS fkc
INNER JOIN sys.foreign_keys AS fk
ON fkc.constraint_object_id = fk.[object_id]
INNER JOIN sys.tables AS t
ON fkc.parent_object_id = t.[object_id]
INNER JOIN sys.schemas AS s
ON t.[schema_id] = s.[schema_id]
INNER JOIN sys.columns AS c1
ON c1.[object_id] = fkc.referenced_object_id
AND c1.column_id = fkc.referenced_column_id
AND c1.name = N'MyTableID'
WHERE fkc.referenced_object_id = OBJECT_ID('dbo.MyTable');


Results:

dbo    OtherTable1    FK__OtherTable1__ID__32E0915F
dbo    OtherTable2    FK__OtherTable2__ID__35BCFE0A


And even generate a script to drop and re-create them (dropping the redundant unique constraint in the meantime):

DECLARE 
  @sql1 NVARCHAR(MAX) = N'', 
  @sql2 NVARCHAR(MAX) = N'ALTER TABLE dbo.MyTable DROP CONSTRAINT myx;', 
  @sql3 NVARCHAR(MAX) = N'';

SELECT 
  @sql1 += N'
ALTER TABLE ' + QUOTENAME(s.name) + '.' + QUOTENAME(t.name)
  + ' DROP CONSTRAINT ' + QUOTENAME(fk.name) + ';',
  @sql3 += N'
ALTER TABLE ' + QUOTENAME(s.name) + '.' + QUOTENAME(t.name)
  + ' ADD CONSTRAINT ' + QUOTENAME(fk.name) + ' FOREIGN KEY '
  + '(' + QUOTENAME(c2.name) + ') REFERENCES dbo.MyTable(MyTableID);'
FROM sys.foreign_key_columns AS fkc
INNER JOIN sys.foreign_keys AS fk
ON fkc.constraint_object_id = fk.[object_id]
INNER JOIN sys.tables AS t
ON fkc.parent_object_id = t.[object_id]
INNER JOIN sys.schemas AS s
ON t.[schema_id] = s.[schema_id]
INNER JOIN sys.columns AS c1
ON c1.[object_id] = fkc.referenced_object_id
AND c1.column_id = fkc.referenced_column_id
AND c1.name = N'MyTableID'
INNER JOIN sys.columns AS c2
ON c2.[object_id] = fkc.parent_object_id
AND c2.column_id = fkc.parent_column_id
WHERE fkc.referenced_object_id = OBJECT_ID('dbo.MyTable');

PRINT @sql1;
PRINT @sql2;
PRINT @sql3;
-- EXEC sp_executesql @sql1;
-- EXEC sp_executesql @sql2;
-- EXEC sp_executesql @sql3;


Results:

ALTER TABLE [dbo].[OtherTable1] DROP CONSTRAINT [FK__OtherTable1__ID__32E0915F];
ALTER TABLE [dbo].[OtherTable2] DROP CONSTRAINT [FK__OtherTable2__ID__35BCFE0A];

ALTER TABLE dbo.MyTable DROP CONSTRAINT myx;

ALTER TABLE [dbo].[OtherTable1] ADD CONSTRAINT [FK__OtherTable1__ID__32E0915F] 
  FOREIGN KEY ([ID]) REFERENCES dbo.MyTable(MyTableID);
ALTER TABLE [dbo].[OtherTable2] ADD CONSTRAINT [FK__OtherTable2__ID__35BCFE0A] 
  FOREIGN KEY ([ID]) REFERENCES dbo.MyTable(MyTableID);


This explicitly handles this case, where the constraint only involves a single column. It gets a little more complex if there are multiple columns involved (and this answer is not meant to solve that problem). I also didn't test if this works exactly as coded if the foreign keys point to a redundant unique index (which has the same underlying structure but is created with slightly different DDL). Exercise for the reader. :-)