The Desire 530 looks to be low on specs by today's standards, especially with 1.5 GB of RAM. If you have a lot of apps trying to run in the background and run out of space, the system will start closing apps so it can make room for another that needs to initialize for whatever reason
Basically it's the same symptoms as what can happen with memory optimizers, only it's the OS itself doing it. Android was designed to run with most of its RAM in use, but it's a finite amount.
Hard to say if that's what's going on here, but something to consider.