Don't use just one word in this case. Frequency of what? Reproducibility, rarity, don't feel natural and occurrency isn't even a word (not that you to care ;). Go with bugFrequency
or get more detailed if you want. Long variable names are not a crime, especially if you actually plan to keep the code around for more than a few milliseconds.
Also, do you have any particular reason for wanting to use enum style values instead of just integers? Since you'll probably often want to sort the bugs, enum like values for frequency doesn't really make sense. Some global integer constants of when you define bug to be 'rare' or a 'common' should suffice in your categorization. This lets you eliminate a the additional enum field entirely and just do this:
if bugFrequency < RARE_BUG_MINIMUM_FREQUENCY:
//treat as rare
else if bugFrequency < UNCOMMON_BUG_MINIMUM_FREQUENCY:
//treat as uncommon
else if bugFrequency < COMMON_BUG_MINIMUM_FREQUENCY:
//treat as common
instead of checking for an internal enum value. Since you'll use multiple factors to judge the importance of the bug, enum values don't make as much sense at this point.
(sorry if this was way more of my opinion than you wanted :)