DriasIngresAppSvcPlan 100% cpu,high memory, hosting web apps stopped reacting

Hello

today, at 10:00 our resource App Service Plan DriasIngresAppSvcPlan started 100% cpu,high memory, hosting web apps stopped reacting.
This AppSvcPlan hosted four Web Apps. Restarting then didn't helped. Scaling up to higher plan didn't helped.

I created new plan DriasIngresAppSvcPlan2 and moved all Web Apps on it. Now web apps are working.

But I wan't to know what happened? Can this happens sometimes in future? This is very dangerous for us, because that Web Apps are data collectors and when they go offline, it leads to data loss.

Finnaly the strange thing is, that there is no web app on DriasIngresAppSvcPlan but this plan still occupies 80% memory. Why? Its empty.
I would like to move web apps back to the DriasIngressAppSvcPlan, because there are configured alerts. But can I do it whet the plan has so much memory occupied and is empty?