Failure (closing delays) are not an option, unless the Buyer wants their Earnest Money to become "permanently non-refundable"!   If they close late - their EM is still their funds - but - they pay for any expenses resulting from the delay....

Sometimes the appraisals, as the appraisers are slammed (as we are), sometimes the buyer is slow to get required documentation to the lender, sometimes it is the lender asking for last minute documentation.

(Okay, looks like duplicate question Roy Kelley )

  underwritiing asking for things at the last minute

Lenders not doing what they need to do in time.

financing

Buyer having last minute financing conditions that have to be met are the biggest reason for closing delays.