Yes that's likely the cause as long as there's no clogging in the inlet restricting flow
Why not rinse sand? It's just as able as before the rinse regarding bacteria: cleaning by rinse Isn't anti bacterial. It would take medications or harsh cleaning to kill its filtration ability.
Lastly, unless this is a qt the sand filter is only a benefit in the mind. In a reef tank and in a fish only tank of common design, surface area isn't lacking ever/ so even if you ran the filter totally empty, no parameter you can measure would change
These filters are indicated solely in very high bioload setups with reduced decor and surface area in the overall flowpath design
What kind of tank do you run it on?
In 99% of setups you get the same tank benefit disconnecting it permanently as you do running it. The sole benefit of the mechanism is + surface area for ammonia control. It would require a dense stocking ratio of fish, long term, in a barren tank to require that type of filter. In a reef tank or any tank with rock in it/100% neutral device. No harm in running it, no benefit either other than the current it produces from its output.
You can rinse the sand in common drinking water from a container/5 gallon jug of you want and it will not sterilize the media. Limited timeframe osmotic changes do not kill biofilm housed marine filter bacteria.
