This happens every time you receive enough EVO points to level up when shooting. I noticed this while playing as him and it's very disrupting with weapons that have slow handling like the Rampage, and can ruin a good firefight easily with unfortunate timing.
Attached are four test scenarios - two from the sling (Rampage+R301, bug happens with both), one perfectly normal R301 scenario (no bug) and one non-sling Rampage scenario during the ultimate (no bug). From this I concluded it only happens with sling weapons. In all of these clips I held down the ADS button and fired in a similar manner. I also tested with the R301 to ensure it's not just the Rampage that's bugged, seeing that it had a similar issue with the rev-up being lost at the exact same point earlier.