Why is react converting my dropdown into a scrollable box when I add the multiple attribute to <Form.select> tag?

<Form.Group className="mb-3" controlId="formBasicEmp">
              <Form.Label>Manager</Form.Label>
              <Form.Select aria-label="Default select example" class="selectpicker" name ="f_manager" value={this.state.f_manager} onChange={this.handleFormChange} multiple={true}>
      <option> Choose manager
      </option>
    {managerOptions}
    </Form.Select>
            </Form.Group>

Before adding multiple attribute

After adding multiple attribute