To perhaps expand on Sawdy's comment, I'm not so sure it's a matter of the media picking things up. At least from my anecdotal perspective, the amount of people being killed in any singular act of violence has certainly upticked. Whereas in the past you would get a lot of shootings each night that are not reported on the local televised and national news, and they were almost always one person killed and maybe another person injured who was either too close or not the intended target. The mass shooting that happened here in Santa Ana (not too far away from me at that) was certainly a disturbing upgrade over the typical personal vendetta shooting in that the murderer targeted several coworkers along with the owner. Clearly it's a mass shooting, but, at the same time, it's not like the mass shootings we're accustomed to seeing where the assailant has a grudge against the institution rather than the victims. We might not have realized mass shootings are considered "4 or more" because this is the first time I can recall where mass murderers were targeting multiple people rather than just inflicting collateral damage.
As to why that is, I think it certainly bears study in addition to what is already being investigated about mass shootings. It's entirely possible that the pandemic created a different strata for those who might have means but aren't necessarily staying afloat financially. Such a person might be someone who lost a job during the pandemic, but still has enough savings to survive short term and has skill set that is marketable in non pandemic settings. There is room for resentment in such a group that would transfer laterally to others in the same overall socioeconomic strata instead of upward. That might explain how more people are specifically targeted in shooting.