The city had a master bricksman or brickster or whatever you call him, and not a master paving stone guy?
The city just had one guy working, and by the time he got to the paving stones, he was too tired to do a good job?
The paving stone guy was angry cause the brickmeister was smoking (see butt) and decided to sabotage the project?
There's an undiscovered fault underneath the street and a minor tremor caused the paving blocks to get skewed?
Some malcontent with a grudge against Mayor Fraim came by that night and took a crowbar to the paving blocks?
Bricklayer, that's it! Okay, maybe the master bricklayer got annoyed because the paving stone guy kept calling him a 'brickster' and smacked him on the forehead with a trowel and gave him double vision, which caused him to misalign the paving stones. I'm going with that one.