I've had this happen before, more in the past when I used to ride a couple hundred miles a week, on average (road and mountain miles combined)... I've been out of the saddle for years, so I can't maintain that kind of pace anymore!
Sometimes the bonk would hit in the middle of the ride, sometimes at the beginning, sometimes just randomly at anytime. I used to have a set training schedule, so that I would know in advance when I was going to ride, and most of the time after warming up (3-10 miles for me) I'd be at my most ready - my most metabolically efficient state. I used to be very vigiliant about carb-loading the day before a ride, which I still recommend -- not doing so, for me, would almost always result in poorer performance in general and less endurance. I used to cross train on the road for stamina, and would ride 50 or more miles at a time. Doing so requires *a lot* of calories, and without sufficient nutrition, I would have bonked for sure on many of the rides I undertook.
That being said, there were a number of rides, perhaps 5% to even as much as 10%, were I just would not be at peak energy, for no particular reason. It was not that I'd changed my routine or ridded too much the day or two before -- these were just random incidents of not feeling the same energy and endurance level that I was used to. Occasionally, I'd even have to bail out of a ride early, and stop on the way home to have lunch or dinner. Pretty much invariably during these early bail-outs, I'd find myself starving with a capital S - I'd be much more hungry that I would have expected.
I chalk up those experiences to something with the performance of my liver, though to be honest I have only a rudimentary understanding of human biology or its metabolism -- I'm no scientist for sure

Regardless of that fact, I've heard layman talk about how the liver serves to process carbohydrates and to make energy available when it's needed. On the converse side, sometimes I would find myself with unexpected reserves of energy, and I would end up turning what I expected to be a short ride into an epic event, lasting 4 hours or more... with the sore ass to prove it afterwards!
Lastly I would suggest that it's very important to warm up, and to make sure you make time each and every ride for some intervals. I've found nothing else superior to intervals for extending and enhancing my overall endurance and stamina, both in terms of while muscle strength, or sprinting, non-aerobic power, and the red muscle tissues, which of course are at the core of sustained aerobic effort. But I can't stress the warm up enough -- it's just not natural do go from sitting in a car to mashing your big ring five minutes later.
~Dacob