c# – Entity with compound primary key also create index on single column

I just added an EF entity called PurchaseOrderProduct, which creates a many-to-many link between my PurchaseOrders and Products.

[PrimaryKey(nameof(PurchaseOrderId), nameof(ProductId))]
public class PurchaseOrderProduct
{
    public int PurchaseOrderId { get; set; }
    public PurchaseOrder PurchaseOrder { get; set; }

    public int ProductId { get; set; }
    public Product Product { get; set; }
}

However, I don’t quite understand the generated migration script. I want the primary key to be a compound key based on the PurchaseOrderId and ProductId columns. But why does it also create an additional IX_PurchaseOrderProducts_ProductId index on my ProductId column?

/// <inheritdoc />
protected override void Up(MigrationBuilder migrationBuilder)
{
    migrationBuilder.CreateTable(
        name: "PurchaseOrderProducts",
        columns: table => new
        {
            PurchaseOrderId = table.Column<int>(type: "int", nullable: false),
            ProductId = table.Column<int>(type: "int", nullable: false)
        },
        constraints: table =>
        {
            table.PrimaryKey("PK_PurchaseOrderProducts", x => new { x.PurchaseOrderId, x.ProductId });
            table.ForeignKey(
                name: "FK_PurchaseOrderProducts_Products_ProductId",
                column: x => x.ProductId,
                principalTable: "Products",
                principalColumn: "Id",
                onDelete: ReferentialAction.Cascade);
            table.ForeignKey(
                name: "FK_PurchaseOrderProducts_PurchaseOrders_PurchaseOrderId",
                column: x => x.PurchaseOrderId,
                principalTable: "PurchaseOrders",
                principalColumn: "Id",
                onDelete: ReferentialAction.Cascade);
        });

    migrationBuilder.CreateIndex(
        name: "IX_PurchaseOrderProducts_ProductId",
        table: "PurchaseOrderProducts",
        column: "ProductId");
}