I'm guessing it's just the cold weather (35F, raining) that I've been training in lately, but I have bonked twice at the end 1-hr rides this week. My training recently jumped from ~6 hrs/wk to ~8 hrs/wk because of some fast commuting to a new campus (riding at high SST, near FTP). So it must be a combination of increased training load, not much dietary increase, and cold weather. I'm also losing weight.
Yesterday I even ate a Kudos bar 20m before I headed out yesterday. I've been 3 hrs from my last meal on departure for the last 3 years, and not bonked until this week, on rides up to 1.5 hrs.