Symptoms
Though the rules are not configured, Open Tasklist rule will be auto fired when there are parameter called "SN" or "SerialNo" in form parameter.

Diagnoses
This only happen when there are parameters called "SN" or "SerialNo" being passed on to runtime forms.

Resolution
It appears that SN/SerialNO are reserved words. The Runtime Site picks that up as you are attempting to open the Form and follows that code path.