For years, I've bought avocados 4 or 5 at a time from Costco (or the grocery store when they were on sale). I've always let one or two soften on the counter until ready, and refrigerated the rest. Very occasionally they would go from rock hard to rotten-seeming, with large black or gray-brown areas taking up 1/4 - 1/2 of the flesh, and with no "ripe" stage in between. Lately, though, this seems to be happening much, much more. The last 2 or 3 Costco batches I bought did this for every single avocado. It doesn't seem to matter whether they were the refrigerated or non-refrigerated individuals; either the whole batch is good or bad. What gives? Is this an issue with the fruit quality or my storage/ripening practices? If the former, are there signs I can look for when buying avocados to predict if they will be good or if they'll rot like this?

tldr; Why do my avocados go right from rock to rotten?

EDIT: Thanks, all, for the great suggestions.
If you're someone else coming here for help with this issue, here's a summary of the comments so far:

• Avoid knocking, bumping, dropping, or otherwise abusing the fruit. The cause of all this may be bruising. Much of this is out of our hands and in that of the store.

• Pre-ripe refrigeration may not just retard ripening until you take out the avocado to ripen, but may prevent things from ever ripening. Let ripen and then refrigerate if longer preservation is needed.

• The above conflicts with the observation some have made that avocados are nearly always shipped/stored refrigerated and the real problem is when they are frozen (or perhaps refrigerated too cold / for too long). [Editorial comment: I'd love to see a source comparing the veracity of this claim to the one right above].

• To speed ripening, ethylene works like with many other fruit -- seal the avocado in a brown paper bag with apples or bananas and it will ripen in a day or two.