Honda - Cr-v :: 1999 - Radiator Overflowing Coolant Fluid

1999 Honda CRV. Took it in for an oil change last month, then next day took a long road trip. After the first half of the trip, I opened the hood and saw colored fluid sprayed all over the front of the engine. It seemed to be coming from the top of the radiator. I assumed the common failure mode of these radiators, ie that the joint between the plastic top piece and the aluminum radiator body was beginning to fail. The overflow tank was empty, but the radiator was still full.

But before I drove the return trip home, I replaced the radiator cap with a new one just in case. When I got home, I checked the engine compartment again. No more coolant spray. So I thought the new radiator cap had solved the problem.

So I added fluid to the overflow reservoir, up to the "minimum" line. A few days later after some local driving, checked again. Again, coolant sprayed around, seemed to be coming from around the cap, overflow reservoir empty, but radiator still full.

Now I just leave the reservoir empty, instead of filled to the "minimum" line. The radiator is not leaking, because I've checked the level several times since and coolant is always near the top of the neck. The only problem is when I add fluid to the "minimum" line in the reservoir, which results in coolant being sprayed out (apparently through the radiator cap, both the old cap and the new one). Now I suspect the oil change mechanic also checked my fluid levels and added coolant to the overflow reservoir, which resulted in the original "spray" episode during my highway trip.

So the question is, why is this happening all of a sudden? Never did this before. Now I just leave the reservoir empty and check radiator level occasionally. But I would like to know why any fluid added to the reservoir when cold causes the radiator cap (apparently) to blow out the excess coolant (it is NOT leaking from the overflow reservoir).