We present a detailed quasinormal mode analysis of gold bowtie nanoantennas, and highlight the unusual role of the substrate and the onset of multi-mode behaviour. In particular, we show and explain why the directional radiatiave beta factor is completely dominated by emission into the substrate, and explain how the beta factors and quenching depend on the underlying mode properties. We also quantitatively explain the generalized Purcell factors and explore the role of gap size and substrate in detail. These rich modal features are essential to understand for future applications such as sensing, lasing, and quantum information processing, for example in the design of efficient single photon emitters.

9 pages, 7 figures