One thing about ISFj's, and I have read this in several places on the forum is that often times they are ruled by their insecurities:

I feel your pain. I really do! ISFJs are my supervisee, so I want to help them. Help them not be afraid of everything. Help them relax. Help them blah blah blah. I've had two ISFJ girlfriends, one I chucked pretty quickly, and another one that really fucked with me over the course of a year. They're damaged goods if you ask me. I don't usually make sweeping judgement calls, but every ISFJ I've met has been ruled by their insecurities. They're nasty lying shit-talking fakers. But I too love a challenge, so by the time i realize just how damaged they are, it's too late. Argh.
And just as often their behaviour makes little sense especially as introverted feelers, they might even seem bi-polar.

And that's perhaps the explanation for the sometimes perceived so-called "bipolarity" of ISFjs: they are assuming that everything is under control and fine, so they have no barriers at all; then something happens to show that no, it's more "chaotic" than they thought, so they so an U-turn and erect those Se barricades for worst-case scenario. For an outside observer, those U-turns may look bipolar.