Suction leak

[mas985]

That sounds like your filter is filling with air which is creating the very high back pressure. Air is compressible but water is not so if there is a lot of air in the filter and the pressure is fairly high, it will create a lot of back force when the pump is shut off and can easily blow a lid off a pump.

Normally, filters will self purge the air via vent pipe in the filter so you might what to check that. Here and here are videos that shows what can happen when there is too much air in the filter.

The next step is to figure out where the air is comming from. As I posted before it is usually an air leak from the suction side of the pump which will draw in air when the pump is running. The small air bubbles could be an indication of this although the filter should be able to purge that amount of air if the vent tube is clear.

However, another cause is a leak in the backwash valve gasket so the filter will suck in air when the pump is off. It may not leak water when it is on either so it isn't always easy to tell. However, when the pump is shut off, you can hear the water draining and air bubbling through the filter so that is usually a good sign that the filter may have an air only leak.

[ChuckGeo]

If the you are only seeing a small amount of bubbles in the pump and getting bubbles out of the returns sporadically, it may be that when the bubbles go through the pump and into the filter they build up in the filter, then escape through the auto air relief in the filter and out the returns. If you are getting air in the pump you need to concentrate on the suction side of the system, downstream of the pump is under pressure and won't let air in the filter.
Does your vacuum work OK? Does the air in the pump increase when you hook up the vacuum? You may not have a big enough problem to worry about, or you may want to call in a pro for help.

[rww]

I had a similar situation. Turned out to be aleak in an o-ring in a Pentair valve just before the pump.

[maybe]

do you have a automatic suction pool cleaner? if so unplug it and see if the air bubbles go away, if they do check the hoses.

when theres an air leak it's from skimmer to pump thats it anything beyond pump would leak water not suck air.

[bsw]

This is pretty old but...An Ozone Generator will create larger bubbles in the gasket and create an abundant amount of small bubbles usually through one and not all of the returns. At least that is how mine operates.