Makes sense.
IIRC, the Titanfall games pretty much only used Source because it was what they used to prototype early on, but then indecision, delays & other studio issues meant that they had to stick through with what they already had. Unreal Engine was one of the other engines in consideration at the time.
I suppose if they were working to pitch the SW title to EA, they needed something to prototype again, and would not have had access to Frostbite pre-studio-buyout.
I believe they had insomniac'' engine but due to license reasons had zero starting assets so even prototyping was slow.
Whilst trying to define what the game should be like a dev made a prototype in source as he was familiar with it and progress was much much faster. The team liked it when they saw it on the Monday and this made them re-evaluate their current engine choice.
I think time frame mostly led the final decision, most of the team was familiar with it and it was proven on last gen consoles to deliver 60fps with low input latency. I think insomniac's engineengin by the wayside due to the extra time it would have taken to use. Unreal was not proven on PS3 at 60 I don't think .
There is a posr mortem of the whole dev process that details engine choice and shows early prototypes. It also goes into the Xbox One and how it ended up an exclusive.
http://finalhoursoftitanfall.com
It's a paid app but it's good content from what I recall.