As mentioned by Ivo Jansch, Andi Gutmans (of Zend) responded to the recent speculation about a buyout of the company in his keynote address at this year's php|tek conference:

Andi explained the reason: they want to become cashflow positive, in order to remain independent. The news should be interpreted in a positive way, the future now looks bright, Zend products will not be affected and life goes on. An acquisition of Zend is not happening in the near future.

TechCrunch had suggested that the recent large amounts of layoffs could have been in an effort to ready the company for a buy out from one of many proposed major players.