Apparently, comments containing really long URLs go into Moderation: it’s as if the program’s just too lazy to read it all and figures “Yeah, there must be something bad in there somewhere.”
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Comics I Don't Understand 
It could be the sheer length, as you suggest. Another possible factor (and which is demonstrably relevant to whether an image URL will simply give an embedded image as often desired) is whether the URL is a simple classic path, or whether it contains a lookup or query processing item (generally indicated by a question mark).
Given all of the general oddities in moderation, I’m impressed that you managed to notice one specific one among the chaff.
Mitch4, nothing embedded just a simple really long URL.
And Arthur, I really only noticed it because it happened few times lately. Otherwise, as you say, I’d just figure “It happened because it’s Saturday.”
This is a security feature.
One of the things the bad guys do is try to get software of their choosing to run on your computer (or, your host’s computers). One of the ways this can happen is called a buffer overflow attack… they intentionally send more characters than the system is designed to handle, and the last ones just happen to be code that, if executed, gives them something they shouldn’t have. Some software will actually overflow the buffer (the space set aside to hold something) and then keep writing, and if the circumstances are just right, this allows them to write code into a place in memory where the program will actually transfer control to the data that was just written. It isn’t SUPPOSED to happen that way, but sometimes it does. Modern CPUs have hardware assistance to prevent this sort of problem, but plenty of older servers continue to serve… particularly in web-host farms, where computers are replaced a few thousand at a time, and the computers are in the five-figure range per each.