First All Grain batch completed - Question about runoff

I just completed my first all-grain batch and I have lots of questions about things that just didn't quite go as expected. Rather than list all questions in a single thread - I thought I'd just open a thread for each question.

First thing that happened was during the sparge/runoff. I was doing a 10 gallon batch and when I put everything into beersmith , it had me fly sparging with 9.6 gallons. So I had everything going at a nice pace - keeping an inch of water above the grainbed, it looked like the whole sparge was at a pace to complete in 60-70 minutes or so. I was supposed to capture 13.4 gallons but when I got to about 12 gallons, the water level was below the grainbed and it was just trickling out of the drain valve. I then opened the valve all the way and it was still a trickle.

I then very gently pushed on the top of the grainbed (and i do mean very gently). Then my son moved the drain hose a little and we noticed it was gushing out. It came out so fast that it actually filled the brew kettle up to 14 gallons (past out 13.4 gallon target). It actually did that last 2 gallons in about 2 minutes.

If I were thinking a little clearer - I probably should have shut the valve a little to slow that down.

So - I'm not sure what all of the sudden made the draining go from a trickle to a gush. Could there have been some kind of airlock in the hose? Or could have pushing on the grainbed have done anything?

I suppose that I should descibe the mash tun. It's a 70 qt cooler and I was using a copper manifold with slits dremeled into the pipe every half inch or so.

So any insight would be greatly appreciated.

Thanks in advance

I have beersmith as well, I learned no matter what to always aim 1.5 gallons above what it tells you for sparge water. As long as you do that and ONLY lauter what beersmith says so and you wont have a problem hitting your gravity.