I am betting it was slightly low on coolant in the radiator, and when the coolant got hot it boiled, letting off steam into the coolant tank, which in turn broke due to age/being brittle.
Have it replaced, the coolant levels checked, and then have them run / idle the bike to the temp than the fan kicks on, and see if it does it again.
Also, when you get the bike back, double check all fluid levels yourself. I don't trust dealerships really... (too many bikes passing through to care).
1986 Kawasaki Vulcan 750
NGK Iridium Plugs #7803/DPR7EIX-9
Duralast Gold ETX15 AGM Battery
Coastered & Shaved
Metzeler ME880 [110/90-19, 170/80-15]
Balance Dampers Replaced
Rebuilt Forks w/ Progressive Springs
VN750.com Grill Cover
Emgo 23-92411 Handlebars
MOSFET FH012AA R/R
1990 Kawasaki Vulcan 750
1998 Honda VFR800 FI
2014 Honda VFR800F
1989 Pontiac Firebird Formula 350