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This is a 2 x 2 mm chip with 12 no lead contact points. Half of the time I get it out of the reflow oven, it's twisted like in the image below.
I am using a stencil in order to lay the paste down and then tweezers for placing the little chips. There is no fan in the oven and I let the boards drop to < 170 C (leaded solder paste) before pulling them out. There are also some even smaller 0603 parts which never twist / tombstone.
How do they become crooked, and what can I do in order to avoid it ?
This is most likely caused by the long exposed pads.
In the oven, all the paste doesn't melt at the same time. Those long pads that flow first have an enormous amount of surface tension and can pull the chip out of alignment before the rest of the paste has a chance to flow.
You can try to fix this with a temporary peelable soldermask:
After applying your paste with the stencil, wipe off the excess as close to the chip as you dare (perhaps 1 mm). Apply the soldermask over the pads you wiped free of paste, then put your chip down.
As Chris Stratton meantioned in the comments, kapton tape could also work. Depending on the type it may leave a residue, but you can probably just leave it applied.
