Two general reasons have been cited for LTC insurance premium increases by [some] insurance companies.
1) Lower policy lapse experience than was actuarially projected by [some] companies for their LTC insurance products.
2) Extremely low bond rate environment. High grade bonds are one of the most common investments that insurance companies use to back the insurance pools for insurance products. By law, insurance companies have to maintain a relatively low risk profile for the investments they use to back their insurance products.
Here is a 2010 MorningStar video that discusses the issue.
Defense Tactics for Rising Long-Term Care Insurance Rates
The low bond rate environment is a challenge for other institutional investors too (e.g. pension funds, other types of insurance products, endowments, etc)