Film dielectric breakdown, possibly from over voltage condition, high peak current or over temperature. When a cap breaks down, hydrogen is generated inside. PC mounted caps have "vents" (grooves) in their tops that are supposed to let this pressure release before the rubber bung on the bottom blows out. Sometimes that doesn't work out. Many axial lead(as used for point-to-point wired components), and multi-section capacitors are built like a small tin can with a cap crimped into one end. When pressure builds up from electrolytic breakdown in one of those, you often get the firecracker effect. A tube with an internal short can cause a sudden high current condition. Hopefully it's that simple.