Why Coax loss comes from several practical sources, and all three listed items add to it. Water in a connector or inside the braid changes the dielectric and adds heavy dielectric and resistive loss, often ruining the cable permanently. High SWR means power reflects and travels back and forth along the line, so the same energy passes through the lossy conductor and dielectric multiple times, multiplying the matched-line loss. Every connector and adapter adds a small amount of insertion loss and a possible impedance discontinuity, so stringing several in a run adds up.
Watch out Each single choice is a genuine loss source, so picking just one, such as high SWR, is only part of the picture. When a Technician question lists three plausible physical causes, the combined answer is usually the intended one.
Water, watts bouncing (SWR), and connectors: all three steal signal. Seal your PL-259s.