Why would a pin-hole develop?

[MonaxLisa]

I was putting something under the sink when I heard a mysterious hissing sound. Turns out one of the black rubber hose type pipes under there developed a pinhole and was spraying a tiny stream of water. Everything is soaked. So I shut off the water outside since someone trying to be helpful lost my owners manual and I don't now how to turn off just that part of the plumbing. Honestly you wouldnt think the space would be big enough to loose something. Anyhow! Why would that happen? I mean...it just seems odd. What else can possibly go wrong with this thing, aghh!

[1bbboomer]

My 78 Excella did not have rubber hose connections or shut of valves.
My water lines were copper tube with soldered joints.
Leaks happen, but at least the rubber connection lines are fairly easy to replace if you can get to them.
I replaced all of my lines with PEX and installed shut off valves just in case.
Like I said, leaks happen.

[4RXLA]

Do you have a "dishscrubber" coming out of your counter? My 77 does and it has a retractable rubber hose.

[Fyrzowt]

It is not at all unusual for a leak to develop in a 1975 trailer, for many reasons.
The plumbing may have been stressed in times past due to poor winterization, vibration from road trips, a thin spot in the wall of piping during manufacturing, etc. The possibilities are endless.
I too recommend installing Pex type tubing whenever you do a repair. Either replacing all of it at once, or in sections as needed(my personal choice). Pex stands up to vibration and freezing much better than the copper that came with the trailer in 1975.
Good luck.
Dave