Showing new cards in random order

[abdo]

You can also change the deck setting to ordered mode and back to random mode to resort the cards.

Randomizing Anki cards

From the manual: Order controls whether Anki should add new cards into the deck randomly, or in order. When you change this option, Anki will re-sort the decks using the current option group. One caveat with random order mode: if you review many of your new cards and then add more new cards, the newly added material is statistically more likely to appear than the previously remaining cards. To correct this, you can change the order to ordered mode and back again to force a re-sort. So the “s…

I guess what you want would require recalculating order each time you add a new card, which could be bad for performance.