Pages cursor jumping

I have a Mac running OS 10.10.3.
I'm preparing a document in Pages (into which I've inserted many photographs). When I try to type text, the cursor/point of insertion suddenly jumps to another part of the document. What might be the problem?

Does it do that with other applications or just with Pages? Also, if you're using a MacBook or MacBook Pro (you didn't mention which machine you have) you may be accidentally leaning or brushing the trackpad causing the cursor to jump. It's easy for that to happen while you're typing.