Q:

What happens when a Mystic runs out of disciplines to learn? 

The most recent version (v3) of the Mystic class from Unearthed Arcana can be found here. 
The Order of the Nomad gets 2 additional disciplines at 1st level from the "Bonus Disciplines" feature, meaning that all of the Nomad disciplines are unlocked by level 5.  
However, at level 7 and above, the number of disciplines known increases further for all mystics.  Normally, I would think that the mystic simply would unlock nothing, but at level 20, the sheet says that all mystics know 8 disciplines, which is more than any one order has.  
What do you do if your mystic has learned all the disciplines for an order, but is still able to learn more disciplines? 

A:

Only the bonus disciplines from your order need to be from your order's disciplines
In other words, you are free to choose any psionic discipline from any order, except for the two bonus disciplines you get at level 1 from your order. 
From Psionic Disciplines, pg.  3:

At first level, you know one psionic discipline of your choice.  The Disciplines Known column of the Mystic table shows the total number of disciplines you know at each level; when that number goes up for you, choose a new discipline. 

There is no mention of restrictions, so there aren't any.  Compare that to, say, the Bonus Disciplines feature from Order of the Nomad (they all have similar wording, I'm just picking this one as it's in your question), pg.  7:

At 1st level, you learn two additional psionic disciplines of your choice.  They must be chosen from among the Nomad disciplines. 

Yes, it does also say "of your choice", but it specifically calls out the restriction of being from your order. 
Also consider the Order of the Soul Knife.  It doesn't have any psionic disciplines associated with it, nor does it get these two bonus disciplines (which every other order does) at 1st level.  However, an Order of the Soul Knife Mystic would still get to learn disciplines as per the number of known disciplines listed in the Mystic table; this is because Mystics can choose anything when the level up, the only exception is their order's bonus disciplines (if they're anything other than an Order of the Soul Knife Mystic).