- changes in hydraulic conditions (clogged pipes, clogged filter, ...)
- pressure switch failure
- flow sensor failure
(in normal condition the pump starts when the consumption is higher than 1.2 l/min or the pressure is lower than 2 bar)
https://www.manualslib.com/manual/1569763/Grundfos-Mq-3-35.html?page=1#manual
If the old hose isn't leaking, why change it. They discolour internally due to the type of water you have.
Chalky water makes white calcium deposits and soft water more brownish. Perfectly normal.