Virulence of mycorbacteria is due to:
Friends there are multiple factors are listed as virulence factors of the MTC, few of them are:

Lipids and Fatty Acid Metabolism and Synthesis of complex lipids, lipoproteins
Mycolic acid synthesis
Cell envelope and cell wall proteins
Some secretion system
Proteins Inhibiting Antimicrobial Effectors of the Macrophage and protein kinases
Different Proteases and proteins associated with it
Apoptosis inhibition and arresting the phagosome
Mycotoxin production
Cell envelope
Resistance to inactivation by heat
Proteolytic bacteria