There could be various reasons why a shop request gets declined, and it's a very common occurrence.
The most likely reason is that the shop was assigned to another shopper - in that case, the requests for that shop from all other shoppers are automatically declined. The schedulers are making the decisions of whose requests are approved, based on several factors - primarily the requested date and the shopper badges.
Schedulers also sometimes decline requests even only one shopper has requested the shop - if they are confident that they can get the shop scheduled soon and do not wish to assign the shop to that shopper - it can happen if the requested date is too far away, or if the shop was requested for the same day when the request was made (schedulers can then think that the shopper would not have enough time to prepare adequately for the shop), or if the shopper that requested it is not sufficiently experienced, etc.
I hope this provides some clarification :)