I find that these five images tell an interesting story of a team's progress. The students had never worked together before, and most of them had never been part of an agile team. They were learning not just technical skills but also professional and social ones. The charts illustrate this learning during the semester, as they improve with their abilities to estimate, to plan, to deliver technical results, and to respond to changes.
That bump in Sprint 2 is interesting. The team recognized that many problems in the first sprint came from a combination of procrastination and poor estimation. They started in on their tasks right away in Sprint 2, and when things didn't go as well as hoped, they blew up the estimated hours remaining on those tasks. It turns out that the tasks weren't as bad as they feared, and as they worked through their problems, the estimates dropped back down to more accurate levels.
The erratic path in Sprint 5 illustrates that the team underestimated the effort required to polish the games for public release. Interestingly, the burndown chart was still useful even though the estimates were quite wrong. The team could look at the chart, which said they were dozens of hours ahead of steady, while knowing that there was still significant work to be done. This discordance forced them to re-evaluate their tasks and estimates, to reconsider the sprint's plan, and eventually, to successfully deliver on every user story.