Elevation, of course, is a big factor. At "normal" elevations, it's typical to boil off about a gallon over 60 minutes, but in Colorado, I would expect this to be closer to maybe 1.5 gallons or something like that (I'm not sure). But if as you say, this is the first time you were so far off, maybe this wasn't the biggest factor for you. Just something to be aware of in your boiloff estimates.
Another big thing is the wort lost in the hot and cold break. Is it really 0.75 gallons? For me it's much less, for others it could be that much or more.
And in similar fashion, what about hops? If you normally don't use a ton of hops, but this time around you went crazy and used 12 ounces of hops or something like that, they will soak up a TON of wort, especially if you used whole hops instead of pellets. This can knock your volume way down.
Humidity is another one. Your boiloff rate will be huge if the humidity is only 20%, versus very slow on a muggy summer day when the humidity might be 90%. This can have a big impact.
There are probably dozens of other factors. We don't have all the information here on the web, so you will need to take the time to backtrack every single step of the way to nail it down. And even then, it might not be easy.