Annual Cast iron baseboard explosion

I have a two story with half cast iron baseboard and half fin and tube; the two on different zones (pre-existing).
When I moved in four years ago, the kitchen radiators (12 sections) were cut off at the basement and left ww an open pipe because one along the interior wall was cracked. I replaced that section and reattached the length to the system.
Each year for the last four years, at least one section of the two-foot radiator peices has busted open. Each time I drain the system, take the full length apart, rent the connection tool and replace the section. Afterwards I bleed what I can access and await the next burst. Usually the breaks occur in the kitchen and are only hairline cracks along 9-inch rust-ridden iron. The one last winter was an obvious sign of expansion and along an exterior wall (assumed from a freezing draft). This was already the fourth section in that room; two along an interior wall and another along the exterior. So during the summer we had the house fully insulated with foam and blown-in cellulose. We also had the Weil-McLain Gold boiler professionally converted from oil to natural gas.
Then during this fall at the start of the heating season, one broke in the bathroom along an interior wall. I replaced all of the 6 shorter sections with two 9 inch against the exterior wall only because the shorter ones are no longer available.
Last night during dinner another kitchen section exploded, splashing us all at the table and hitting the ceiling (no injury).
The house psi is 30. The boiler is set at 120L - 140H. Boiler gauges read 142 F, 50 psi and 110 feet H2O. There is a relief valve rated at 150 psi which allows water flow when manually opened. There is an expansion tank attached above the boiler.
What In the world is happening?