getting HR up into zone

I've heard others talk about this before but experienced it for the first time yesterday and was curious about it.

I went for a 1 hour run yesterday morning...that got me to 7.3 miles, which I think the Garmin said was 8:15 pace. My HR would not get into my upper zone 2 which is where I wanted to train. I was stuck in Z1 for a long time and had to exert more effort to get into low Z2. Only when I was going uphill did I get it into higher Z2. I think my zones are pretty solid...normally they fit. Usually a sustained pace of 8:30-8:50 gets me into upper Z2.

Then last night I rode the trainer for an hour watching the NBA Conference Finals. Same situation as the morning run. I worked harder than I normally do and could not get my HR up. I know I worked harder because my legs are sore today.

I've had days when I am overtrained, tired, lack sleep and have trouble keeping the HR down, and that I understand. And I know you want to be able to go faster with a lower HR, but this was abnormal.

What causes this?