I lived in Hudson, FL for 5 years and regularly (weekly) fished from Hudson>Anclote Keys>WikiWachee>Clearwater>Tampa Bay and out 45-50 miles in a 23' Suncoast (local FL built) center console walk-around. Generally speaking, there really aren't that many reefs @ the 45-50 mile offshore mark. Tournament fishing we'd drift fish. I never had more than two people (myself and one other) that far offshore - because of the considerable weight difference/shift. Whenever we anchored, it would be only during calm (2'-4' seas); and, then when we'd anchor it was only because we were going spear-fishing on the rock piles. I'm not sure if folks are (possibly) meaning rock-piles rather than reefs (again, there aren't that many reefs @ the 45-50 mile mark).
I was surprised to hear they had anchored and when I did hear they had anchored, I was thinking they must have been spear-fishing the rock-piles. Of course, you don't need much water to capsize, and the water isn't very deep @ 45-50 miles offshore (maybe 50'-60' - if you're lucky).
Even during the rare occasion I've been caught offshore with developing seas, storms, etc - I never really recall the seas ever approaching anything like I've encountered in any other area I've boated - other seas, oceans, bays, etc. IMO, even 45-50 miles offshore from Clearwater the wave period is nearly 8-9 seconds and often even greater - the slope is so unbelievably gradual all the way to shore.
I can't help to think the way they anchored may have been the significant contribuiting factor (in my mind anyway). And what I mean by that is I can't help but to think they had a very, very, very limited scope - 30'-40' - which isn't nearly enough - especially if you're hooked up to a rock-pile (in particular). If you know the seas are running high, I'll generally ensure I have double the scope for depth and seas. Whenever I've anchored on those rock-piles we'd toss out about 200' scope - which as I recall was always more than ample. If seas are tall, and you shorten the scope to less than (and significantly less than the minimum), you just ask to get beaten up. The longer the scope, the greater the ability to weather those tall knarly rogue waves.
In any case, it's sad and a good reminder (for myself anyway) to always toss out the maximum or greater scope on building seas or even during normal seas especially if they're running high.
Very sad.