This just happened to me for the first time last night/this morning. WY2565, 4 liter starter in a 4 liter flask. This flask gas a decent amount if head room, but I guess not enough for 2565.
This was a step up using 2/3's of a crashed and decanted two liter starter. Two liter starter was a touch foamy, but didn't spew.
I guess I should of expected it. In the past 2565 foamed when I looked at it sideways in the fermenter. I was changing blowoff sanitizer for 5 days.