Simple explanation's to why this is happening to you is...

1) You wait to long to pull
2) Once you do pull if you have a hard time keeping aggro just spam flash for the group of mobs. then 1/2/3 the ones that you see are getting focus for dps and you'll hold it. If you have trouble with aoe dps on a large group spam flash and use youre fast/riot blade combo for mp and keep flashing. Should never ever have a problem with these two options.
3) Suck up the fact that they pull and take the aggro back. flash the area of mobs and 1/2/3 the targets they are on, eventually you'll take it and continue on your way.

When I dps and I see tank hesitating I will pull yes, Especially in low lvl dungeons where this is not an issue. So stop hating on the dps and start honing your skills to over come this. It will be a large part of high level play where holding aggro is a big part of fights when dps needs to output.