Grain has quite a bit of lactic acid producing bacteria in it, on top of it's just being readily available in nature in general, so breweries naturally have a ton of it around. Most common lacto and pedio bacteria, the primary two type of LAB, is not very tolerant to hops. If you make a really hoppy beer, this naturally helps prevent the beer from souring.
Of course, a lot of beer is intentionally soured by these, and basically all beer originally would sour, and it's perfectly safe to drink. Just not desirable in every style of beer.
And, if you’re not a fan of extra-hoppy ipas, letting an ipa age will mute the hoppy flavor and could potentially make a more enjoyable beer.
The alpha acids in hops oxidizing will have an impact on the flavor of the beer - largely it will reinforce the malt flavor, but depending on your susceptibility to off flavors, you might get cardboard or BO type flavors.
The only reasonably reliable way I've found to get around this is to buy refrigerated whenever possible, and to look for "what wasnt here last week". Or buy from one of the handful of breweries that does print dates.