Striker Christian Benteke scored twice either side of Andreas Weimann's well-worked goal before Steven Gerrard struck a late consolation as the hosts' three-match winning run was ended.
"We are very disappointed and to be honest I didn't see that coming as our momentum and confidence coming into the game was probably as high as it's been," said the Reds boss, whose side dominated before Benteke's 29th-minute opener.
"We probably got punished for our decision-making in that first 20 minutes, we were a bit careless. We could easily have been 3-0 up in the game and if we do that we can control it much better but we didn't have that final pass and bit of quality at the top end of the field.
"Then we went on to concede some poor goals. We weren't quite at it today with our intensity and pressure in the game and they were a threat on the counter-attack and got the goals.
"I thought we were flat. It was important for us to get the first goal - with them playing 3-5-2 - to open up the game but we had those chances early on and didn't take them.
"Once that happens, combined with sloppy play and giving the ball away in poor areas, we contrived to concede bad goals."
It was the first time Villa had scored more than once in any half of league football this season and extended their unbeaten run to five matches with only a second away win in 17 away league games.
"I think the whole performance was fabulous, the goals were excellent as was the way we defended," said manager Paul Lambert. "I thought we were excellent. The goals were brilliant: the second goal, with the movement and the backheel, especially.
"Benteke has been unbelievable while Andy Weimann knows where the goal is, he's a natural finisher and his enthusiasm and work ethic are second to none."