Below are some things that cause most Google places pages for businesses to suddenly disappear.

• A URL in the review
• 2+ reviews from different accounts made from the same IP address (computer)
• 2+ reviews made from the same network IP address (Ex: two different computers in a building)
• Change of physical address will erase all your reviews (un-recoverable)
• Having the ‘privacy setting’ in Google set to ‘private’ instead of ‘public’
• Over-keyworded review (Ex: the keyword ‘new York car wash’ repeated over 3+ times)
• Frequency of incoming reviews (Ex: 3 reviews in the same day from the same location, even if it was 1 PC and two phones)
• Excessive exclamation marks
• If the same review text is found on another location or another large review site, it may be removed

The short answer: Just be legitimate.