In fact I have seen this happen.
I have 2 theories.
Firstly the temperature that it was set at. And maybe you angled the pipe without realizing and the wet meth fell down the side of the pipe causing it to do this.
Secondly..... Cuts.
I smoked a bit of "Speed" in my younger days before I realised that it must be incredibly detrimental to your health and I found that it set differently. It also took a really long time to set, like a minute or 2. This is all I can think of.
The fact noone else has chimed in suggests to me that this is a rare and abnormal occurance and probably not a good thing.