"I was having a really good ride and he was fairly well up on time," he said. "At Discovery Valley he just over-jumped - I may not have had too nice a stride into it and he just launched and lost his footing when he landed.
"It's not something he has ever done before ... it is just really frustrating. Without that, we probably would have been inside the time."
British rider Oliver Townend and Ballaghmor Class lead the field on 40.6 penalty points, ahead of compatriots Gemma Tattersall (43.0) and Izzy Taylor (45.6).
New Zealander Tim Price and Ringwood Sky Boy sit fourth on 46.5, after picking up 5.2 time faults in the cross-country, while Andrew Nicholson aboard Nereo is sixth on 47.1, after adding 7.6 time faults.
The top six are separated by less than two dropped rails going into the final showjumping phase.
Price was happy with his placing, given that his round with Ringwood Sky Boy was a little scrappy.
"He is not the most elegant, tidy jumper, but he is incredibly enthusiastic and brave, and I try to use those things as our main tools," he said.
Price said a win at Burghley was the ultimate goal.
"It would mean everything to me - I came close a couple of years ago and was fourth last year.
"It has been quite a testing year and to win would be the icing on the cake."
Nicholson had nothing but praise for his chestnut gelding.
"I have a much easier job riding Nereo than most of the other riders have out there," he said.
"He knows every type of fence that is coming and he knows where he should be jumping from. It makes it very easy to ride."
