First, you start calling things "non-lethal aid". Then you start calling weapons "lethal aid". The phrase appears sporadically in news coverage, slowly normalizing itself as a term of art among military-watchers and foreign policy nerds. Then it suddenly bursts into widespread use because of intensive use in military and government communications about "lethal aid" (i.e. to Ukraine). Some media put the term in quotes, because it's a creepy dead-eyed euphemism like "collateral damage" or "enhanced interrogation techniques" and they know it. As time passes, though, media stops putting the term in quotes. After all, everyone knows that it means arms shipments, right? Hey, we're not here to educate readers.
Military euphemisms creeping into headlines is a lot like police press releases about "officer-involved" incidents. The intent is different, but the end is the same (facts less clear to readers) and the means requires the co-operation of journalists who know better but have other priorities (productivity, continued access, editors to please, etc.)