It's because of memory android phone. Android phones run on an algorithm where all old apps which are open recently are save in memory so suppose you are running a phone which has 1 GB of memory and you installed 5 games and 10 other apps and you opened 5 apps while using ODK Collect.
So, now comes on memory management your android OS wants 500to600MB depends on the android phone then 5 apps they will take nearly 100 to 200MB so nearby 80% memory of your phone is already been used.
Now you open ODK and start collecting data then that will also take memory and then your phone will do work like this.
So, the best way to do work in the field gives your field worker android phone with minimum apps those are required and ask them not to install many apps on the phone.