Why did my piece of beef harden on crock pot?
It wasn't cooked long enough. A long cook time in the crock pot dissolves the collagen in the meat and causes it to get tender, but before this happens the meat fibers will tighten (think an egg firming in the frying pan) and the meat will be tougher.

Here's a chart of high vs low cooking times on the Crock-Pot website. As you can see, cooking times on the low setting are around double the times on the high setting. So you can think of your sirloin as having cooked 6 hours on low, which is on the short side.

It's also worth mentioning that that hour set on high was largely spent getting up to temperature, so overall it was probably cooked less than the equivalent of 6 hours on low.

Try setting it to low and leaving it for 8 hours next time if you want to serve it in slices, and more if you want to shred it.

HIGH : LOW

3 : 7

4 : 8

5 : 9

6 : 10

7 : 11

8 : 12

*** It is not recommended to convert recipes with cook times less than 7-8 hours on "Low" or 3-4 hours on "High."