I take it by 'lien' you're referring to a registered mortgage? If so, any registered mortgagee can start the power of sale process. In practice, second mortgagees generally either don't do it or do it with an abundance of caution because the first mortgagee's interest gets paid out as a priority after covering the actual costs of sale. If there's no money left after that, the second mortgagee gets nothing.
The process of sale is also quite prescripted. Although it is intended to obtain fair market value, it often doesn't in practice.