There are four drugs being tested for effectiveness against COVID-19 in the SOLIDARITY trial. Three of them have obvious potential mechanisms of action:

• Remdesivir is an RNA polymerase inhibitor. It might stop the virus from multiplying once it infects a cell.
• Hydroxychloroquine may interfere with the spike protein the virus uses to attach to cells.
• Interferon beta has general antiviral effects.

But I can't find anything that would explain why the Lopinavir/Ritonavir combination is expected to do anything. Both drugs appear to be protease inhibitors that are fairly specific to HIV, while SARS-CoV-2 isn't even in the same general group of viruses as HIV.