What caused "Liliana of the Veil" to double its price during a single week?
Magic cards rise in price when the pro players start analysing the new sets and proposing that certain cards will be key players in the new Standard.  I believe Liliana has attracted a lot of favourable press, and has appeared in some successful "Solar Flare" decklists at big events lately: of course people are going to jump on that bandwagon, in case she turns out to the be the next big tournament all-star Planeswalker.

Having said that, a cautionary word from channelfireball's Traderous Instinct column:


  Liliana of the Veil
  
  Over the past few days, Lilly has started making an end run toward
  Jace, the Mind Sculptor prices.
  
  She’s out of stock both here and at Star City, and if you want her on
  eBay you’re going to have to spend at least $55.
  
  If I had one for trade at the moment, I would be asking for around $70
  worth of stuff in return.
  
  While many people immediately point to Jace as the obvious comp for
  Liliana, a better comparison is Tezzeret, Agent of Bolas. That card
  debuted around $45 retail, and had fallen to around $30-$35 by the
  first weekend it was tournament legal. The deck performed well, and
  the card gained $25-$30 in value overnight.
  
  Yeah, people REALLY don’t want to miss “the next Jace.”
  
  Just like Tezzeret, though, Liliana is REALLY narrow. She is currently
  seeing Standard play, and I don’t expect that to change, but she
  doesn’t draw cards, bounce creatures, lock your opponent out of the
  game, and then win the game like Jace does.
  
  The thing is, though, Magic players like to have fun – even tournament
  players. And for many Spikes, fun is roughly equivalent to playing the
  most wonky, slow, toolsy control deck that it’s possible to crush the
  opposition with.
  
  Much like mono black control, people REALLY want Solar Flare style
  decks to be good. So when two show up in the top 16 at an open, game
  on!
  
  Even if Solar Flare is tier-1, there’s no way Liliana holds a $60+
  price tag for long. She’s only there because she’s a planeswalker and
  we still haven’t really figure out how to value those yet, five years
  after their inception. I recommend that you sell on Liliana right now
  and buy back in when the hype has died down somewhat.