Gaggia Classic. Portafilter coming off!!

Happened a few times over the past few months.
Was doing my head in.
So, bought an unpressurised basket for a few euro. Problem solved.

But..........why was it coming off mid brew and making a mess in the first place?

Grain size to small? Tamped too tight? These are the things I imagine were happening but no, not really, tamped lighter and went "slightly" coarser, still, occasionally coming off.

Google didn't provide too many answers except........
....a few Yanks on some forum were saying, ditch the pressurised basket, which I did and ditch the "perfect cream pin" which I did.

Things have settled down and nice crema minus plastic pin to boot.

But.....why did and does the portafilter come off? Too much pressure I imagine?

Cheers.