Perhaps a sign that gun laws in this country are working well enough that others are turning to edged weaponry? Additionally, look at some of the places where these events transpired: New Jersey, Texas, Florida... Texas and Florida, given their hispanic population booms, I can understand. But New Jersey? Perhaps it's not a coincidence that NJ has some really strict gun laws? Just a thought. Take it FWIW.
I think it's because they are illegals and can't buy guns legally, and are leery of buying them on the black market, because they suspect it might mean quick deportation if they are picked up while carrying. OTOH, a machete can be explained away as a "yard work tool." Until it's choppin' time.