Being transgender is not relevant to shootings, but there are voices that are trying to make that happen.

My opinion on why it gained traction: the group is already marginalized, is part of a larger, also marginalized group (lgbtq community), and shootings are unpopular, while guns are, so it benefits the speaker to connect the two. There are also narratives floating around that are in synergy with this connection, such as the tragic statistic that trans people have a very high suicide rate, and the false narrative that being transgender is a mental illness.

https://www.snopes.com/fact-check/transgender-mass-shootings...