Why is "感恩" frequently used as a verb? In the last two years or so I began to see people frequently using 感恩 as verb meaning "thank (you)", such as "感恩大家" or "感恩领导". 

But I thought "感恩" is a verb-noun phrase, meaning "thank (you for your) kindness", while "感谢" is the phrase for "thank (you)". 

Is the use of "感恩" instead of "感谢" a newly emerged usage, or have the two always been interchangeable?
You are right that 感恩 is a verb-noun phrase. When another noun is appended (i.e. 感恩XX), it means that you are thanking (感) someone (XX) for their kindness (恩). Inversion is at play here. "感恩" carries a degree of formality and is rarely used in daily conversation. It will be awkward if someone uses "感恩" when not speaking publicly.To rate them in terms of formalities: 感恩 > 感谢 > 谢谢 > 多谢感恩 is used exclusively in formal context while 多谢 is only used casually. 感谢 and 谢谢 can be used at the user's discretion, though the former sounds more formal.