It shouldn't leak out of the cap. The bottle has an overflow tube and excess coolant should escape through there. Maybe it's clogged?
If you hadn't added coolant recently and it started spitting it out, chances are you are low on coolant. It can get low enough to boil and purge coolant via steam/etc if the bike gets hot enough.
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