how was your perceived exertion? HR is fickle and subject to lots and lots of "noise". As others have stated it could be cumulative fatigue from prior days/weeks training, hydration, weather, nutrition, amount of sleep or rest or time spent on your feet non-bicycling that contributed.
I've had workouts like you're describing when training with HR as primary metric, and found it frustrating. Also had workouts when using HR where my heart rate was soaring straight through target zones effortlessly for no real reason I could identify. That's a reason that I took the HRM off last year and just trained with sensations, speed, critical distance, and this year am using power + those other metrics, but find myself still primarily relying on sensations. Don't read too much into it, it's one day. Take a few zone 2-ish days, hydrate, focus on sleep/rest, eat well, and you'll be flying again in no time.
Also, if you're really overloading yourself, you'll have days where you just cant hit your target HR or power #s, usually happens to me at end of a 3 week build, then you know it's time to chill for a few days.