Tajikistan is a quite poor authoritarian country with a rapidly growing young population (It was ~5M in 1991, ~10M now) and under the huge influence of drug lords trafficking heroin from Afghanistan (it is around 1/3 of their GDP if you include shadow economy).
Russian huge (compared to Tajik) economy was the main stabilizing factor, with many, many young Tajiks working in Russia not only removing unemployed young men from the country but also bringing a nice influx of cash those workers send home.
Now, with the recession in Russia (caused by some mysterious reason...) this stabilizing factor is vanishing. It may explode any moment... And I think the local dictator tries a small victorious war against a neighbor to prevent this.
Kyrgyzstan is in a way similar but it is not on a major drug trafficking route and it is a relatively democratic country with frequently changing governments (even if transitions are often... quite violent) they also have growing population but to a lesser degree (from ~4.5 in 1991 to ~6.5 now). They are also less dependent on Russia, with their spare workforce going not only to Russia but also to Kazakhstan
Also, note that Tajiks and Kyrgyzs aren't closely related culturally. Tajiks speak Farsi and Kyrgyz language is from Turkic family. Don't think about "stans" that they are closely related nations.
I predict that this region WILL explode one way in another. Americans leaving Afghanistan is one destabilizing factor, the Russian economical crisis (and loss of respect towards their military might) is another.