So Corona's parent company (including Negro Modelo, Pacifico) resists the price increases that Anheuser-Busch sets and Miller generally conforms with (collusion?). This has increased their brands' market share. So with the underperformance of Bud Lime (how could that have failed?), AB looks to buy a controlling share of Modelo (AB owns 50% already, which seems odd to be threataned by a company you have that much equity in). But the DOJ is thinking anti-competitiveness. As for the millions of micro-beers? They are a fragmented market, and don't factor in to the anti-trust equation. Modelo sells 4-5 well-known brands, about $7B revenue, so it's no small potatoes, but is still considered a maverick in the highly consolidated market.
I could care less about beer, but economics is cool. Listen here: