Well, the replacement tire also cracked, and the front is showing rather alarming (though probably not dangerous) sidewall cracking too.As long as the tire's not bulging, it is not likely to blow out; but the cracks can let road grit in, which will chew at the threads and also possibly work its way into the tire and cause a flat. (I've had the latter happen to an old pair of Ritchey Tom Slicks.)
My rims, Ambrosio Excellences, are superb: light and incredibly strong, they haven't needed a touch of the spoke wrench since I got them some 17,000 miles ago--but they are very narrow--only 13mm across the inside, according to the manufacturer's website.
The ETRTO tire/rim compatibility chart does not recommend 28mm tires on rims that narrow. So, I bought a pair of 25mm Marathons yesterday, to see what happens. Will I get to wear out the tread before they crack? Is it rim width that causes the problem, after all? Or is it the relentless UV exposure they suffer in LA? And does anybody out there want some 3/4 worn Marathons for skidding or for grocery runs? They're excellent tires, except for this one issue.
Stay tuned; we should know in a year or less.