We were poorly organised as a team in that situation. We didn't seem to agree on what we were gonna do from the kick-off. Kane and Alli sprinted forward to press, the defenders dropped back, wary of a long ball over the top and the midfield was somewhere in the middle. This put us in a situation where there was too much room between the attack and the midfield and more importantly in this case, between the midfield and the defence.
Then when the long ball came, Davies made a schoolboy error by challenging in the air against their center forward (Vardy), leaving his own man (Mahrez) unguarded behind him. When Davies didn't clear the ball, he was out the equation. Vertonghen then needed to cover, and the central midfielders and Chadli didn't get back in time to help him out because the gap between the midfield and defence was too big.
So... a combination of poor organisation and an individual mistake.