$20K to $34K over 10 years averages 5.4% per year. According to College Board data on public 2yr, public 4yr, and private 4 yr colleges, tuition and fees increased from 2006-7 to 2016-17, on average, 4.5%, 5.2%, and 4.1%, respectively. (These are all compounded rates and the tuition/ fees are enrollment weighted)
So the extra debt is likely, due mostly to tuition increases, with public 4 yr colleges being the worst offenders.
FYI - Using simple averages debt went up 7% and tuition/fees 5.5%, 6.6%, and 5.0%. Still close, but compounding is more accurate, since this year's tuition increase is always based on last year's tuition.
Source for college board data is below, but you need to look at the current dollar tuition/fees from Table 4 in the excel data. The PDF and presentation are all corrected for general inflation while the original Fed presentation on student debt, that this article is based on, appears to use nominal dollars, i.e. dollars uncorrected for inflation.
https://trends.collegeboard.or... (See download box in upper right.)