First post here:)
Could also be from a previous crit heal from a discipline priest. Divine Aegis looks much more 'bubbly' than PW:Shield does.
As a druid tank, I prefer not to get shielded on the initial pull. I'm not all that knowledgeable yet on the mechanics. I know I get rage from dodging, critting, but I also believe I get some from being hit.
As a rule, when I heal on my priest, I tend to not bubble the tank, except when moving (4 horsemen for example) or 'oh crap, tank's going to die'. The protection of the shield + glyph, has saved a few wipes.

