Why do some mutagenic N-nitrosamines not require metabolic activation?

Dr. Rachael Tennant presented the excellent poster in the 44th ACT annual meeting. She explained some N-nitrosamines show mutagenicity both with and without S9 activation either due to the presence of additional toxicophores or properties that allow for spontaneous decomposition or alternate metabolism.
I believe it helps understanding the mutagenicity and carcinogenicity of nitrosamines.