That looks likE parasitic inductance from your probe stevie .
with this 555 i use it always does that when I use this cheap home-made probe .
When I ise my pro tektronik 10x probe it never does that however .
I am 100% positive that this is due to parasitic inductance , your first pulse is negative voltage after the falling edge ....
It is also not good for components , I used to fry my cmos when I used my homemade probe . Your problem seems quite severe .
