Why would a different law enforcment agency (different city) be in a different jurisdiction?

I was driving home in Los Angeles' jurisdiction and I saw glendale PD driving through the intersection with sirens on. Glendale is at least one city away so im confused as to why they would be there. as i turned through this intersection i saw them turn around and go back the way i first saw them coming from. anybody know why they might be in LA jurisdiction?