Three hundred watts at 12 volts is 25 amps. That's a fair bit of current. Maybe you're getting some corrosion which would cause increased resistance, which in turn would cause excessive heating.

You might try using lower wattage bulbs. If it were my system I would probably do away with all the crimp-on connectors. I'd solder and tape all connections, then coat with liquid electrical tape.

Brickeeye might have some better ideas than mine.

Sure, water intrusion could have caused your problem.
water certainly conducts, shorts out , = burnt connection.

18v with water will burn off a 18ga center conductor on RG6 cable.

fred

18v with water will burn off a 18ga center conductor on RG6 cable.

RG6 is an RF cable. It isn't meant to be used as power cable, certainly not for landscape lights.

FYI, pure water doesn't conduct electricity. But this is mostly theoretical, since pure water isn't common in nature.

That is quite a bit of current and really does need a good quality connection. The picture appears to indicate corrosion that may be from minerals in the soil (or sometimes well water) or from products contained in fertilizers or other lawn/garden treatments.

A better quality connection that doesn't use pierced type connectors would most likely solve your problem.

The use of low voltage DC power also accelerates the corrosion when water gets in, and it WILL get in.

As noted there is a good deal of corrosion that accumulated over a long time from water, garden chemicals, etc. The cause has to be removed so check the location for ponding water, etc. Good connectors are certainly important but the best ones won't overcome intrusion of that much water and/or chemicals.

In a situation like that, if I had to make a splice, I'd probably twist the wires securely, solder them, and then insulate with tape and sealer.

Solder is outstanding for excluding moisture and air (and thus corrosion) from splices, but it has to be applied correctly.

The correct heat shrink splices for the individual wires will create as good a seal as you can make while creating a soldered joint.

Some quality rubber splicing tape with an overcoat of vinyl electrical tape should make a decent long term splice.