Walmart is notorious for coming into an area and running at a loss until it has eliminated all the competition then jacking up prices.
1. Walmart opens location
2. Prices things to get a small margin
3. Competition is driven out because consumers don't want to go to multiple stores to get everything they need
4. Suppliers raise costs, inflation happens, market conditions change, etc. (it would even make sense for the supplier to raise prices if they have less independent customers, to account for the risk of only having 1 big customer)
5. Someone writes an article about how X is more expensive now at Walmart than it was at random shop, years ago.