Perhaps this will answer your question:
A started service can use the startForeground API to put the service
in a foreground state, where the system considers it to be something
the user is actively aware of and thus not a candidate for killing
when low on memory. By default services are background, meaning that
if the system needs to kill them to reclaim more memory (such as to
display a large page in a web browser), they can be killed without too
much harm.
More info can be found here