To add on to Glorfindel's answer:
"Dump" has an underlying meaning
to release, store, or discard a large quantity of something
related to these definitions from Merriam-Webster
3 : to sell in quantity at a very low price, specifically : to sell abroad at less than the market price at home
4 : to copy (data in a computer's internal storage) to an external storage or output device
Definition 3 describes the action of releasing a large quantity of a product into a particular market at a drastically reduced price. This forces the competition to also lower their prices, possibly below the point where they can sustain their business. Through this practice an unscrupulous manufacturer might take a short-term business loss in order to capture a monopoly on a particular market. It's called "dumping" because the products are sold in large quantities, (since small amounts would have no effect).
Definition 4 describes the storage of a relatively large amount of "raw" data related to some computer process. Because raw data is not filtered in any way before the "dump", it needs to be parsed afterward to find anything relevant. For example, Facebook might to a "dump" of the logs of all user activity over a certain period of time, which can then be separately analyzed to find particular patterns or locate one significant record.
In a similar way, an "adrenal dump" means that the gland releases a significant quantity of the hormone in response to a particular stimulus. The use of "dump" implies that the release is somewhat unregulated in quantity -- that the gland simply releases as much as it can.