This will be the reason:
Ebay doesn't allow regular sellers to simply suspend a listing temporarily. If an item has a long history of many sales, but stock runs low, the seller will change the price to keep the last one from being sold and losing all the sales history.
The sales history figures in the eBay search algorithm. Products that have great success are very much liked by eBay. It means fees! Thus, eBay are keen to keep these products high in the search results.
When a listing drops off, you lose your sales history. When you list again, you start from scratch.
Putting the price up to an unrealistic value will dissuade buyers, allowing the listing to remain and keep the sales history intact. When new stock arrives, the quantity can be updated and the price changed.