Had to be a bit frightening for the performers on the higher floats. But very unusual that this caught them by suprise. I thought all the parks has tech backstage to tell if any storms or severe weather patterns were on the way. I think the Park took a risk since to cancel a show or parade is the last option.
We had a very similar down pour like this a couple of weeks back at the Studios Park in Paris during the Stunt Show. Yet since it happened at the start of the show. Most of the stunts were kept in and the show carried on like normal.
I think the rain was caught in the middle on the parade, so they had no option to just end it the way it is, because I know they have "rainy day" versions of parades (but still... this is a heavy rain)
DLR: 2001, 2002, 2005, 2006, 2009 may
WDW: 1993,1994, 1995, 1997, 1999, 2000, 2004, 2006, 2011
DLP: 1996, 2002, June 2009
TDL: July 2008, July 2010
HKDL: wishing it will be soon