http://www.neopets.com/ntimes/index.phtml?section=editorial&issue=241

"I've heard that by donating to the money tree, your can increase your chances of a random event. Is this true? If so, when donating neopoints, does it go by the amount you donate or the number of times that you donate? I also heard that taking items from the money tree can decrease your chances of a random event. Is that true? ~ im_102_years_old

None of that is true at all. Random events are just that... random. :) The ONLY way your chances of receiving Random Events is increased is when you're using the Birthday sidebar theme, and that's only available on your or Neopets' birthday."