Tempers flared after City had conceded a third goal and the Foxes fan came onto the pitch to remonstrate with the keeper, who reacted angrily by repeatedly shoving the spectator until he fell to the ground before being bundled away by stewards.
A mundane first half had given no clue to the drama that was to come after the break when, in addition to the extraordinary Walker incident, Villa scored five goals in just 18 minutes.
It was a stunning collapse even by the standards of Leicester who have made a habit of shipping goals by the hatful.
James Scowcroft and Marcus Bent started up front for City in place of the suspended Paul Dickov and injured Les Ferdinand, while Steve Guppy made his first home start since his return from Celtic.
Nolberto Solano went straight into the Villa side after his £1.5million move from Newcastle, while Mark Delaney and Thomas Hitzlsperger passed late fitness tests.
The first clear cut chance fell to City after five minutes when Dion Dublin failed to control Walker's long clearance to allow Bent a clear run on goal, but he shot over from 15 yards with Scowcroft was well placed in the centre.
Nikos Dabizas hauled down Darius Vassell to concede a dangerous free-kick on the angle of the penalty area, which Solano curled onto the roof of the net.
Villa went close after 21 minutes when Peter Crouch headed a deep free-kick from Gareth Barry back across the face of goal, where Gavin McCann was first to react but could only crash a close range volley against the underside of the crossbar.
Leicester hit back through a Guppy free-kick which Dabizas headed over when Ben Thatcher was better positioned behind him.
Vassell then cut inside and rounded Dabizas and John Curtis before unleashing a 16-yard shot, which the diving Walker did well to palm out for a corner.
Villa began the second half in the ascendancy and were rewarded with the lead in the 50th minute.
Hitzlsperger timed his pass to perfection for Vassell to spring the offside trap and send a low curling shot from 20 yards beyond Walker's despairing dive.
City had a chance to level after 55 minutes when a Guppy corner picked out Bent whose downward header was comfortably saved by Thomas Sorensen.
It was a costly miss because two minutes later City found themselves two behind.
Hitzlsperger floated a corner to the far post where Crouch got in front of his marker to head home his first Premiership goal since scoring at Chelsea 20 months ago.
City were dead and buried on the hour mark when Villa raced into a three-goal lead.
Solano's cross from the right was chested down by Barry for Vassell to superbly lash the ball home from 15 yards.
It signalled the clash between Walker and the disgruntled fan.
And there was further misery for the home fans to endure on 64 minutes when they fell four goals behind.
A Solano corner wasn't cleared and with the ball appearing to be drifting out at the far post, Dublin stuck out a boot to squeeze the ball home from a tight angle.
Leicester were now in tatters and Villa completed the rout after 68 minutes, claiming a fifth goal when Mark Delaney crossed from the right for Crouch to turn and crash a 12-yard shot beyond Walker.
Villa could have had more, but City were having no luck themselves when late on Guppy saw his back heel cleared off the line and then Craig Hignett twice blazed over from close range.
Man of the match: Darius Vassell terrorised the Leicester defence all afternoon and deserved his two goals.